The contamination of sediments by organotins poses a threat to marine biota that may last long after release of the substances. The determination of tributyltin (TBT) and its decay products in sediment cores allows elucidation of concentration and degradation trends, and is a useful tool to support management decisions. In this study, organotin speciation was performed on cores from several locations in Guanabara Bay, Rio de Janeiro, to investigate contamination trends over the last 30 years in this area, which houses the second most important harbor in Brazil. TBT concentration in surface sediments ranged from 742 µg kg −1 (as tin) in the vicinity of a major shipyard to 14 µg kg −1 (as tin) in an environmental protection area. Organotins depth profiles were, in general, very irregular, lacking evidence that TBT degradation occurs at appreciable rates in these anoxic sediments. Decay most probably takes place predominantly in the water column and at the water-sediment interface before final burial in the sediments. Data from the least contaminated area was used to estimate a first-order degradation constant of −0.37 years −1 for dibutyltin.

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