The dissipation of the reaction energy and its possible consequences in catalysis on supported metal crystallites are considered. The modes and times required for energy dissipation can influence reaction rates by processes involving excited intermediate molecules at the surface and by producing locally high catalyst temperatures. The boundary resistance at the metal-crystallite-insulator-support interface is shown to play a significant role in determining the temperature-time behavior of a catalyst crystallite. Calculations indicate that crystallite temperatures may be several hundred degrees higher than that of the substrate. A distinct size effect is also predicted with small crystallites attaining much higher temperatures than large ones. The latter may be important in determining the stability of catalysts toward sintering.