A method has been developed to determine cathodic reaction rates and the corrosion kinetics at individual substrate UO2+x grains on the micrometer scale using scanning electrochemical microscopy measurements. By filling experimental probe approach curves to numerically simulated curves, "standard" rate constants for the cathodic reduction reaction on the UO2+x grains are determined. Because the cathodic reduction is coupled to the anodic behavior of the UO2+x at the corrosion Potential, these constants call be used to determine the corrosion kinetics of the UO2+x substrate. The results quantitatively demonstrate that corrosion rate constants vary over a broad range (3 x 10(-6)-10(-3) m s(-1)), and that this range is determined by the degree of non stoichiometry of the grains and the diversity of structures on the UO2+x surface. Our results suggest that corrosion will enhanced at nonstoichiometric sites, probably located at grain boundaries in the spent nuclear fuel, leading to higher rates for the release of radionuclides from these locations. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.3046112] All rights reserved.