The thermal desorption of tritium (3H, T) and radiocarbon (14C) from spent activated carbon was investigated and three thermal desorption steps were established: the vaporization of homogeneously condensed molecules, the desorption of molecules physically binding with the carbon surface, and the decomposition of chemisorbed molecules. A model-free kinetic analysis was conducted to establish the optimum condition of vacuum thermal desorption. Physisorbed species, including tritiated water (HTO) and 14CO2, were effectively removed by vacuum thermal desorption. However, a fraction of 14C, which may take the form of carbon molecules in pyrocarbon form during the heating process, was not removed, even at a high temperature of 1000 °C under a vacuum of 0.3−0.5 Pa. Oxidative peeling of the pore surfaces by filling the evacuated pores with pure oxygen via vacuum thermal desorption and heating to 700 °C was found to be effective for reducing the level of 14C to a level below the established free-release criterion (1 Bq/g) when treating spent activated carbon with 14C radioactivity levels of 162 and 128 Bq/g. The reactivation of the spent granular activated carbon (GAC) by vacuum thermal desorption followed by surface oxidation was also confirmed by the slightly enhanced pore volumes when compared to those of virgin spent activated carbon.