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Harbour sediment contaminated with tributyltin (TBT) is a worldwide problem and treatment of TBT contaminated seawater is an issue during dredging operations. This study presents results from photolytic and photocatalytic treatment experiments where the influence of the seawater matrix, different types of high and low pressure ultraviolet (UV) lamps and different reactor designs have been examined. The study showed that the photolytic rate of degradation in seawater was reduced by 41% compared to the degradation in demineralized water. Moreover, the photocatalytic TiO2 surface was inactivated and produced radicals were scavenged by the relatively high salinity and content of organic matter in the water. Within the same type of lamps, the rate of removal was dependent only on the specific amount of energy supplied to the water regardless of the specific power used. The high power medium pressure Hg lamp was found to be the most efficient compared to the four low-pressure lamps, and the energy consumptions were estimated to be in the 7–8 Wh L−1 range per log reduction of TBT. This study has demonstrated a feasible method for abatement of TBT in seawater matrix.