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For the purpose of assessing the potential influence of tropical rain forest conversion to rubber plantation on catchment hydrology, a stable oxygen isotope was used as a tracer to identify the event water and prestorm water components of storm runoff during low- and high-flow conditions in a native tropical rain forest catchment (TRFC) and an artificial rubber plantation catchment (ARPC) in SW China. Isotopic hydrograph separations for the storm of April and September 2004 reveal that the event water made up only a small proportion of discharge (29 and 31%, respectively) at peak flow in the TRFC, while it made up the dominant component at peak flow (62 and 69%, respectively) in the ARPC. The analyses suggest that in the ARPC, the universal presence of compacted terraced tracks associated with land management would encourage infiltration-excess overland flow. This isotopic investigation demonstrates that tropical rain forest conversion to rubber plantation would markedly change the runoff generation processes in this area.