A program of individual precipitation events and river water sampling and of water isotopic measurements (ΔD,Δ18O) was carried out during summer 1996 along a northeast/southwest transect of the Tibetan Plateau. The spatial distribution of both Δ18O and deuterium excess (d=ΔD-8*Δ18O) of the precipitation reveals three distinct regions. Simulations with a simple isotopic model and seasonal isotopic variations measured at two extreme south and north locations support our interpretation in terms of different summer moisture origins: (1) South of the Himalayan mountains, the moisture provided by the Indian monsoon has been recycled over the Indian peninsula. (2) Between the Himalayas and the Tanggula mountains the oceanic moisture is directly transported from the Bay of Bengal along the Brahmaptra River valley. (3) North of the Tanggula mountains, the moisture is not provided by the monsoon anymore but by continental water recycling. ¿ 2001 American Geophysical Union