The total carbon stock in the top 60 cm of the soil was 63.4 Mg C ha-1 for the pasture adjacent to the plantations and 66.8 and 79.3 Mg C ha-1 for the 4- and 15-year-old rubber tree plantations, respectively. These values are equivalent to an annual increase in soil carbon stocks of 0.85 and 1.06 Mg ha-1, respectively, and they do not include the accumulation of carbon as tree woody biomass. The soil 13C indicated a relatively fast conversion from the previous C4-C (pasture; Brachiaria-Urochloa brizantha) to C3-C (rubber tree). The results from this study suggest that rubber tree plantations have untapped potential to sequester carbon over a relatively short time period.