The stable isotope composition and the tritium content of precipitation analysed within the framework of the IAEA/WMO Global Network for Isotopes in Precipitation (GNIP) have been evaluated for the Indian stations Bombay and New Delhi representing two different regions of the Indian Monsoon. The rainfall at New Delhi is controlled by north-west moving depressions with diverse routes and variable transit times over the land subsequent to their formation at the head of Bay of Bengal. In contrast, Bombay rains are caused by spatially locked cyclonic vortices on the west coast of India with continuous supply of oceanic moisture resulting in negligible isotopic variation over the rainy months. The evaluation of the long-term isotope data of the two stations provided specific information on monsoon rain formation mechanism and the moisture source of the monsoon rains. The average isotope ratios clearly represent an imprint of oceanic moisture during the summer monsoon and of continental moisture during the winter monsoon period. The rainout efficiency is about 30% in case of Bombay (representative of west coast region) and about 56% in case of New Delhi (representative of north Indian Plains). Furthermore, the isotope data indicate that the moisture for low-intensity rains during the post-monsoon phase originates from the Asian continent.