The stability of dispersed high surface area carbon-supported platinum nano-particle electrocatalysts (Pt/C) was investigated as a function of particle size (mean diameters of 1.9, 3.2, 7.1, and 12.7 nm) and oxide coverage under potentiostatic and potentiodynamic conditions in aqueous perchloric acid electrolyte. A non-ideal solid solution theory was formulated to explain the observed dependence of the equilibrium dissolved Pt concentration on potential, Pt particle size, and oxide coverage, as inferred from cyclic voltammetry measurements. The activities of Pt and PtOx in Pt-PtOx solid solutions were correlated with the oxide coverage and Pt particle size. The theoretical framework was also used to determine the rate constants for Pt dissolution and PtOx formation and reduction. The results from the kinetic model were found to be consistent with the measured Pt dissolution for triangle potential cycles with different upper and lower potential limits and scan rates. (C) 2013 The Electrochemical Society. [DOT: 10.1149/2.018306jes] All rights reserved.