The boron isotopic composition of 5 rain waters and 35 river waters running from the high Himalayas (5050 m) down to the Bay of Bengal were measured by ion microprobe to make a first survey of the boron isotopic fractionations present in a major erosion system. The rains have rather homogeneous B concentrations and isotopic compositions (0.09 6 0.09 mmol/l and 5.4 6 1.7‰) at the exception of the two Kathmandu samples which probably reflect a significant anthropic contamination. The river waters show a large range of B concentrations (from 0.09 to 28.68 mmol/l) and of B isotopic compositions (d11B values ranging from 27.0 to 129.4‰). No systematic correlation between the d11B values and the major ion chemistry of the river waters is present, at the exception of the most B and Cl-rich rivers which have systematically the highest d11B values, likely controlled by evaporite dissolution. After correction for the evaporite contribution (from 9.4 to 64.4% of dissolved B with a systematic decrease southwards) the riverine B concentrations and d11B values are still highly variable (from 0.10 to 5.83 mmol/l and from 25.8 to 24.1‰, respectively). This variation reflects mostly different conditions of silicate weathering (congruent and incongruent silicate dissolution), the contribution from the dissolution of carbonates being quite negligible ('5% of the dissolved B) on the river B budget. At variance with congruent silicate dissolution, incongruent dissolution occurs with a large B isotope fractionation which depends on pH (Dclay2water ' 231.2‰ at acidic pH and '25.8‰ at higher pH). The B systematic suggests strongly that these fractionations are produced over a wide range of pH (from 7 to 9.5) in soils and don’t result from reactions between the dissolved and suspended load in rivers. The net result of Himalayan erosion for the global B cycle is the input to the sea of waters with highly variable B concentrations and isotopic compositions (0.67 and 0.29 mmol/l, 15.0 and 121.0‰ for the Ganga and the Brahmaputra during the monsoon period, respectively).