The chemical composition of water in 13 small rivers of the north-western part of Lake Tanganyika was studied over a 15-month period. The factors influencing the water chemistry were studied by cluster analysis. On the basis of major ion concentrations three groups of rivers could be identified. Ionic differences between the groups depended on source altitude which was strongly related to pH, water temperature, conductivity, nutrient content and weathering intensity of dolomite - and pyrite-containing bedrock. These results are in agreement with the riverine inputs of nitrogen to Lake Tanganyika calculated by Hecky et al. (1991), but are ca. 30% lower than their estimated inputs of phosphorus and silicon.