A model for coalescence/sintering of Pt nanoparticles is developed to analyze particle growth and electrochemical surface area (ECSA) loss measured in aqueous tests and in catalyst-coated membrane and gas diffusion electrode-containing single fuel cells. The model combines a non-ideal solid solution theory for Pt dissolution with the dynamics of particle size evolution considering particle growth by Ostwald ripening and coalescence/sintering. Results from the model indicate that the observed growth in particle size and loss in ECSA in accelerated tests are primarily due to coalescence/sintering resulting from Pt dissolution and redeposition between particles. An enthalpy of dissolution of 49.3 kJ.mol(-1) and an effective heat of fusion of 28.2 kJ.mol(-1) for particle coalescence/sintering have been empirically determined from the measured temperature dependence of particle growth. The model suggests that higher Pt solubility in the presence of oxygen is responsible for enhanced particle growth and ECSA loss in accelerated tests in H-2/air as compared to H-2/N-2. The model also indicates that the dissolution rate constant must be reduced by two orders of magnitude to explain the measured decrease in ECSA loss of the Pt cathode catalyst after 10,000 square potential cycles at 80 degrees C when the relative humidity is decreased from 100% to 30%. (C) 2014 The Electrochemical Society. All rights reserved.