The synergistic influence between H(2)O(2) and H(2) on the corrosion of SIMFUEL (simulated spent nuclear fuel) has been studied in solutions with and without added HCO(3)(-)/CO(3)(2-). The response of the surface to increasing concentrations of added H(2)O(2) was monitored by measuring the corrosion potential in either Ar or Ar/H(2)-purged solutions. Using X-ray photoelectron spectroscopy it was shown that the extent of surface oxidation (U(V) + U(VI) content) was directly related to the corrosion potential. Variations in corrosion potential with time, redox conditions, HCO(3)(-)/CO(3)(2-) concentration, and convective conditions showed that surface oxidation induced by H(2)O(2) could be reversed by reaction with H(2), the latter reaction occurring dominantly on the noble metal particles in the SIMFUEL. For sufficiently large H(2)O(2) concentrations, the influence of H(2) was overwhelmed and irreversible oxidation of the surface to U(VI) occurred. Subsequently, corrosion was controlled by the chemical dissolution rate of this U(VI) layer.