Uncertainty about the H⁺ buffering capacity in tropical rain forest limits our ability to predict the future effect of anthropogenic deposition on the streamwater chemistry. Export ofmajor ions to the stream and the ion-fluxes via rainfall, throughfall, litter-leachate and soil-water pathways were observed to examine the source of streamwater nutrients in a small catchment in Sabah, Malaysia. The streamwater and the ion-fluxes were measured for 3.75 and 2 y, respectively, by collecting water twice a month and setting ion-exchange-resin columns. Streamwater pH ranged from 6.5 to 7.6 and was not sensitive to water discharge controlling base cations. The NO₃^--N, Ca²⁺ and Mg²⁺ fluxes were low in atmospheric depositions (0.6, 0.5 and 0.3 kg ha⁻¹ y⁻¹, respectively) andmarkedly increased in litter layer. The NO₃⁻ flux decreased drastically from subsoil (70 kg ha⁻¹ y⁻¹) to the stream (1.4 kg ha⁻¹ y⁻¹) whereas the Ca²⁺ and Mg²⁺ fluxes were not different between subsoil (38 and 18 kg ha⁻¹ y⁻¹) and stream (30 and 15 kg ha⁻¹ y⁻¹). Neutral pH in tropical streams was mainly due to the base cation leaching with deep chemical weathering in deeper strata, and a rapid decrease in NO₃⁻ leaching from the subsoil to the stream.