In situ aberration-corrected transmission electron microscopy, conducted in forming gas (5% H-2/N-2) at atmospheric pressure and elevated temperatures (up to 800 degrees C), was used to follow the growth of Pt nanoparticles dispersed on a high-surface-area Al2O3 support. These direct observations, facilitated by the use of monodisperse Pt nanoparticles, allowed for an unambiguous separation of the relative contributions of Ostwald ripening and particle migration/coalescence to the overall growth process and made possible the estimation of corresponding energetic and kinetic parameters. Whereas the parameters describing Ostwald ripening were as expected, the diffusion coefficient characterizing particle migration was surprisingly small. Size-selection methods were thus shown to have significant potential for suppressing Ostwald ripening, the dominant growth process, in practical catalysts. (C) 2016 Elsevier Inc. All rights reserved.