This work provides basic information about hydrogeochemical contributions to different pathways of water in a small tropical rainforest catchment (3.5 km2) in Rio de Janeiro, Brazil. Precipitation and discharge measurements were conducted at the basin outlet, and at five other stream sites that were selected according to their hydrological behaviour and lithology. Precipitation and streamflow samples were collected weekly, over seven months, and continuously during one storm event. These streamflow samples were analyzed for Ca2+, Mg2+ Na+, K+ and SiO2. Precipitation, throughfall and topsoil water samples were also collected after various storm events, over six months, within a small sub-catchment and were analysed for Ca2+, Mg2+, K+, Na+ and pH. Results from this research showed that rainfall was acid, with low cation content. Throughfall was neutral and cations enriched. Analysis of topsoil water indicated that litter was an important source of Ca2+ and K+ by means of saturation overland flow. In relation to throughfall, streamflow output was characterized by lower concentrations of K+, Ca2+, and Mg2+. Biotite gneiss weathering was the main source of Mg2+ to streamflow. During stormflow periods SiO2 and Na+ concentrations decreased due to Hortonian overland flow contributions to the stream channel. The increasing concentrations of Ca2+ and K+ were due to litter leaching, especially in the zones producing saturation overland flow. Streamflow hydrogeochemistry was not only a product of weathering processes, but also of biotic processes.