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This study investigated a new method for determining fire severity/intensity using FITR spectroscopy and chemometrics on sedimentary charcoal and applied it to two charcoal records from Thirlmere Lakes, located in the Blue Mountains World Heritage Area in eastern Australia; a 135-ka BP record spanning two glacial/interglacial cycles at Lake Couridjah, and a 900-year record encompassing the pre- and post-colonisation of Australia by Europeans at Lake Werri Berri. The larger aims of this thesis were to develop a tool for determining fire severity/intensity of past fire regimes and examine the long-term fire history of Lake Couridjah to investigate how and when (and if) people used fire to change the environment as a consequence of changing climatic and environmental conditions. It was also applied to Lake Werri Berri to examine the transition between Aboriginal and European custodianship of the area around the lake and to determine if their markedly different methods of land-use are apparent in the charcoal record. A new method for determining fire severity/intensity in past environments using FTIR spectroscopy and multivariate statistical modelling (Partial Least Squares regression, PLS-R) is introduced in Chapter 2. FTIR spectra were obtained from a reference library of laboratory produced charcoal derived from common Australian species at Charring Intensities (CI, representing the integration of pyrolysis temperature and duration of heat exposure) frequently reached in bushfires. The modelled coefficients from the charcoal library were then used to quantify the CI of charcoal of unknown heating duration (e.g., modern charcoal, fossil sedimentary charcoal). Although the more homogenous reference library of laboratory produced charcoal is not an exact analogue for charcoal produced in a wildfire, it has the potential to offer insights into the severity/intensity of past fire regimes and may offer an inexpensive and easily applicable tool for evaluating recent wildfires. In the course of exploring Charring Intensity on fossil charcoal, it was found that very little charcoal had CI values below ~3.0°C·s·106 (~400° C). It was hypothesized that the use of a light oxidant, a common treatment for concentrating sedimentary charcoal, was degrading lightly-pyrolyzed material. To test this hypothesis, a series of charcoal samples was produced in a nitrogen purged tube furnace at 250-800 C, then the total surface area of 20 replicates of each temperature group was measured before and after 24 hours of immersion in a 4% bleach (sodium hypochlorite) solution. It was found that charcoal particles formed ≤400 C were almost completely degraded in the bleach treatment and particles formed above at ≥400 C were much more resistant to oxidation. These results suggest that use an oxidant to concentrate charcoal removes material formed at temperatures below ~400 C from techniques commonly used for the analysis of macrocharcoal, with the result being fire histories might be biased toward high intensity (high temperature) fires. The insights from these studies on FTIR and charcoal methods were then applied to the elucidation of the past fire regime at Thirlmere Lakes. This included consideration of the fire history of Lake Couridjah, a record that spans 135 calibrated kilo anni Before Present (ka BP), and thereby encompassing the present and previous interglacial periods (albeit, with a hiatus between ~18-105 ka BP). Importantly, this record covers two climatically similar glacial-interglacial transitions and interglacials (MIS 6-5 and MIS 2-1) ,but notably one was without, and MIS 2-1 had people present. CHAR during both glacial/interglacial transitions was high and variable, followed by interglacial periods of reduced charcoal influx, suggesting that the transitions were times of high fire activity, likely a consequence of climatic instability and increasing biomass accumulation in the catchment. CI, however, was high but relatively stable throughout MIS 6-5, with higher levels often occurring during periods of low CHAR, suggesting that the influx of charcoal into the lake may in part be attributed to catchment stability reducing sedimentation rather reductions in area burned. CI values were often higher during periods of low CHAR, however, suggesting that periods of increased biomass led to higher intensity fires. In contrast to the relatively high but stable CI of MIS 6-5, the transition into interglacial conditions during MIS 2-1 had highly variable CI values that were much lower (~2°C·s·106), suggesting that despite increasing biomass accumulation and a warming climate, the fires were of lower severity/intensity, perhaps the result of human interference in the fire regime. This study suggests that late Pleistocene human populations perhaps attempted to (but ultimately failed) to maintain a more open woodland vegetation structure through selective burning as the ameliorating climate led to increasing productivity in the environment following the glacial termination. At around 10 ka BP, it appears this attempt was abandoned, and CI values suggest that fire severity/intensity increased, and CHAR (area burnt) decreased and remained stable until present, suggestive of a fire regime largely controlled by the stable climate of the Holocene. Five common charcoal analysis methods (4 using an oxidant and one control using H2O) were tested in conjunction with CI and CHAR on a 900-year core (WB3) from Lake Werri Berri. The five treatment groups (4 oxidants and a simple H2O wash as a control) showed very similar peaks in CHAR. However, minimum CI values were significantly lower in the samples treated with H2O, again suggesting that oxidative treatments are preferentially removing lightly-pyrolyzed material. This study considered CI using two combined treatments (H2O and 6 hours in 4% bleach), which captured the largest range of Charring Intensity values (2.8-5.4°C·s·106) and CHAR through the 6-hour treatment of 4% bleach. CI was relatively high and stable throughout the core, with infrequent peaks coinciding with periods of climate fluctuation (~500 BP, late-20th and early 21st century). Interestingly, there was no apparent change in fires (CI or CHAR) during the transition between Aboriginal custodianship and the European occupation of Australia, even after settlement around the Thirlmere Lakes region. It is possible that the people who lived in the area prior to European settlement may not have benefitted from the sustained and systematic application of fire to the environment during the majority of the Holocene, given the resources at hand associated with the freshwater lakes. In contrast, from the latter part of the 20th century, fire frequency and intensity/severity dramatically increased, suggesting that anthropogenic climate change, perhaps in combination with changing land use, caused unprecedented change to the fire regime at Lake Werri Berri, a trend found at a number of other sites in eastern Australia.