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Schwertmannite and jarosite are two of the main secondary iron(III) minerals commonly found in acidic, iron and sulfate-rich environments such as acid mine drainage and coastal acid sulfate soils (CASS). Both minerals exert major influence on the water and soil quality in these environments. While there are many studies conducted on the stability of these two minerals under controlled laboratory conditions, the behaviour of schwertmannite and jarosite under field conditions and the factors influencing their behaviour have not been investigated directly. In the present study, we examined the net transformation of introduced schwertmannite and jarosite samples incubated in a typical acidic CASS environment. Pure (synthetic) schwertmannite and jarosite samples were exposed to two main chemical regimes: 1) aerobic-acidic water column and 2) anaerobic-neutral sediment in a CASS environment. Changes in mineralogy, micromorphology, and composition of schwertmannite and jarosite samples were monitored over a period of 12 months. Schwertmannite suspended in the water column and buried in sediments transformed to goethite by the end of 12 months but more quickly in anoxic, reducing sediments. However, schwertmannite incubated in the acidic water column transformed at a much faster rate than those reported for acidic and aerobic conditions in the laboratory. Jarosite incubated in both the water column and sediments was also transformed to goethite but at a much slower rate than schwertmannite. Dissimilatory microbial reduction and Fe2+-catalysed transformation likely played a major role in accelerating the transformation of both minerals to goethite in sediments. The transformation of both minerals in the water column was sensitive to the hydrological conditions and fluctuations in the water column in relation to antecedent rainfall. In comparison, the sediment's geochemistry was relatively stable and consequently the rate of transformation and dissolution of both schwertmannite and jarosite in this environment was not appreciably affected by variable hydrology. Refereed/Peer-reviewed