The clearing of tropical forest for pasture leads to important changes in soil organic carbon (C) stocks and cycling patterns. We used the naturally occurring distribution of13C in soil organic matter (SOM) to examine the roles of forest- and pasture-derived organic matter in the carbon balance in the soils of 3- to 81-year-old pastures created following deforestation in the western Brazilian Amazon Basin state of Rondônia. Different ?13C values of C3 forest-derived C (-28‰) and C4 pasture-derived C (-13‰) allowed determination of the origin of total soil C and soil respiration. The ?13C of total soil increased steadily across ecosystems from -27.8‰ in the forest to -15.8‰ in the 81-year-old pasture and indicated a replacement of forest-derived C with pasture-derived C. The ?13C of respired CO2 increased more rapidly from -26.5‰ in the forest to -17‰ in the 3- to 13-year-old pastures and indicated a faster shift in the origin of more labile SOM. In 3-year-old pasture, soil C derived from pasture grasses made up 69% of respired C but only 17% of total soil C in the top 10 cm. Soils of pastures 5 years old and older had higher total C stocks to 30 cm than the original forest. This occurred because pasture-derived C in soil organic matter increased more rapidly than forest-derived C was lost. The increase of pasture-derived C in soils of young pastures suggests that C inputs derived from pasture grasses play a critical role in development of soil C stocks in addition to fueling microbial respiration. Management practices that promote high grass production will likely result in greater inputs of grass-derived C to pasture soils and will be important for maintaining tropical pasture soil C stocks.