In situ radiation measurements in the beach sectors and adjacent hinterlands and along rivers in the interiors of southern peninsular India were carried out using a portable radiation survey meter. A very high intrinsic anomalous radioactivity > 26 mu Gy/h has been observed in the hinterlands within a fresh quarry and weathered boulders in the syenite rock body around Puttetti in the western Kanyakumari district of southern Tamil Nadu. Over the weathered hillocks in the hinterlands adjacent to the coast around Inayam, Kurumpanai and Midalam, the in situ radiation measurements have also exhibited high radioactivity ranging from 4 to 22 mu Gy/h which is significantly higher than the radiation exposure rates (RER) observed along the beach sectors at various locations from Chavara to Tuticorin (1-14 mu Gy/h). The observed radiation levels are presumably the highest concentration in southern India and it is the first time that such a high intrinsic radiogenic source in the hinterlands is reported in southwest coast of India. It is also observed based on the laboratory analysis of samples and in situ radiation data that the rivers/channels in this region contain insignificant level of radioactivity concentration and hence they do not contribute much to the placer deposits on the beaches. The placer deposits associated with significant RER (both in situ observations as well as laboratory estimates from samples) in the beach sectors from Kadiapattanant to Inayam are inferred to be derived through the country rocks/weathered hillocks in the immediate hinterlands. (c) 2006 Elsevier B.V. All rights reserved.