The measurements of natural radioactivity due to thorium isotopes have been carried out in estuarine sediments of Mandovi river (Goa). The geochemical behaviour of these sediments has been studied by leaching the samples with 5 % ethylenediaminetetraacetic acid at pH 3.0 in order to investigate the processes occurring on the surface of the sediment particles and the distribution of natural thorium in estuarine sediments. The228Th/232Th activity ratios have been found to be in the range of 2.00 to 2.12. This anomaly between232Th and228Th has been attributed to the preferential leaching of228Ra by water flowing over these sediments. The activities of228Ra on the surface labile layers of the sediments have also been determined. The230Th/232Th activity ratios have been found to be in the range of 0.94 to 1.04. These ratios are mainly dependent on the precipitation action of230Th on adjacent sediments.