The degradation of tributyltin (TBT) in contaminated freshwater and estuarine sediments was investigated for a 330 day period under controlled laboratory conditions. Rates of TBT degradation at different depths were established using regression modelling and revealed TBT half lives ranging from 360-775 d in surficial sediments. There appeared to be little difference in degradation rates between freshwater and estuarine sediments, although a notable increase in TBT half life was evident in TBT spiked sediments. Degradation values suggest that TBT either debutylates to DBT and MBT in aerobic sediments or degrades to DBT which is subsequently desorbed to the overlying water. In anaerobic sediment the half life of TBT was not discernible and appeared to be in the order of tens of years. Biotic processes would seem to be the most important mechanisms for the decomposition of TBT in freshwater and estuarine sediments.