Down core variation of tissue free and organic bound tritium in sediment cores was evaluated to assess the retention of tritium (H-3) by bottom sediment in the dynamic marine environment of Mumbai Harbour Bay. Tissue free water tritium (TFWT) was evaluated by extraction of H-3 from the sediment by extracting it in tritium free water whereas total H-3 by alkaline digestion of core sediment sample. OBT in various core fractions was evaluated by measuring the difference between total H-3 content and TFWT. Total H-3 content in the sediment core varies in the range of 0.41–8.01 Bq g^-1 whereas OBT varies between 0.1 and 0.86 Bq g^-1. Experimental results clearly indicate that total H-3 content in various fractions of the core sediment samples is dependent on the availability of exchangeable hydrogen which were either associated with clay minerals or organic molecule.