Identifying the ripening modes of supported metal nanopartides used in heterogeneous catalysis can provide important insights into the mechanisms that lead to sintering. We report the observation of a crossover from Smoluchowski to Ostwald ripening, under realistic reaction conditions, for monomodal populations of precisely defined gold particles in the nanometer size range, as a function of decreasing particle size. We study the effects of the CO oxidation reaction on the size distributions and atomic structures of mass-selected Au561+/-13, Au923+/-20 and Au2057+/-45 clusters supported on amorphous carbon films. Under the same conditions, AU(561+/-13) and Au923+/-20 clusters are found to exhibit Ostwald ripening, whereas Au2057+/-45 ripens through cluster diffusion and coalescence only (Smoluchowski ripening). The Ostwald ripening is not activated by thermal annealing or heating in O-2 alone.