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This study investigated the thermal decomposition of two structurally similar polysaccharides – cellulose and alginic acid, arguably being the two largest sources of biomass on land and from the sea respectively. Despite the similarity in the two materials and applications, reviewing literature of the combustion and pyrolysis mechanisms of the two materials showed a relatively well-established framework of knowledge for cellulose, although there is still some lack of agreement in some areas. This contrasted with the almost total absence of any information on the thermal behaviour of alginic acid. The aim of the work reported in this thesis was to use the structural similarities and the mechanistic framework for cellulose to conduct a comparative investigation of the two materials. Taking advantage of high throughput multivariate algorithms to process complex spectroscopic data produced by TGA-FTIR, a method was developed to identify infrared active volatile species generated above a detectable threshold during the thermal degradation process. The ability to plot an evolution profile of a selected compound in time- and/or temperature domain significantly enhanced the capabilities of a standard TGA-FTIR system for mechanistic investigations. With the technique established, the two materials were analysed under different heating rates (5, 10, 30, 60°C/min) under nitrogen and again under 10% oxygen in nitrogen. A reconstruction of the cellulose decomposition pathway was carried out using the data obtained from this work. Compared to the methods available in the scientific literature, the current method was a simpler and more efficient technique for providing information related to the thermal degradation behaviours of the sample materials. The application of the method to alginic acid revealed for the first time that this material responded differently and more sensitively to the presence of oxygen than did cellulose. Its decomposition pathway has a number of distinctive steps, in contrast to cellulose.