Hydrometric and hydrochemical monitoring have been carried out at Birthday Creek experimental catchment, Queensland, since 1988. The montane site lies in the Wet Tropics World Heritage Area at an altitude of 1000m, and supports relatively undisturbed rainforest. The area experiences a seasonal climate with monsoon depressions and cyclones bringing 80% of the annual precipitation between November and March. Approximately 40% of incident precipitation is lost as annual evaporation. The catchment soils are highly responsive to precipitation with hydrometric and hydrogeochemical studies suggesting that overland flow and shallow subsurface storm flow are the dominant hydrological pathways during storm events. Nevertheless, the granitic bedrock forms a significant aquifer which sustains baseflows. Streamwater is circumneutral (mean pH=6.66) and the ionic composition is dominated by Na and Cl (concentrations 5.1 and 6.2 mg l-1 respectively). Alkalinity and base cation concentrations are low, reflecting the weathering history of the site. Nutrient concentrations also are low due to efficient forest nutrient cycling. Baseflow chemistry is spatially and temporally constant in the catchment, but streamwater becomes increasingly dilute during rainstorms as overland flow and soil-derived water provide the main sources of streamflow.