The loss of Pt nanoparticle surface area is of great importance when considering the durability of cathodes in polymer electrolyte membrane fuel cells. We here present a model for the loss of Pt surface area that includes both the Pt dissolution/precipitation reaction and the oxide formation/removal reaction. We discuss implications of this model when applied to a distribution of Pt particles under electrochemical potential in both fuel cell and aqueous electrolyte experiments. The importance of particle size distribution shape and distance of the hydrogen crossover Pt sink are explored in detail. We find that particle size distribution shape alters the surface area vs. time curve's functional form and contributes variability to long-term surface area stability. Our model shows that moving the hydrogen crossover Pt sink away from the Pt surface decreases surface area loss and shifts the dominant loss mechanism from mass loss to coarsening. We predict a shift in dominant surface area loss mechanism for in-situ vs. ex-situ experiments (from mass loss to coarsening, respectively) and suggest a critical role for oxide roughening in cycling experiments. The activation energy for Pt dissolution for potential cycling conditions (0.6-1.0 V) is estimated to be 160 meV/atom lower than potentiostatic conditions (0.95 V). (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.011204jes] All rights reserved.