The rate of energy dissipation from Au nanoparticles to their surroundings has been examined by pump-probe spectroscopy. These experiments were performed for particles suspended in, aqueous solution, with average sizes ranging from 4 to 50 nm in diameter. The results show that energy relaxation is a very nonexponential process. Fitting, the data to-a stretched exponential function yields a characteristic time scale for relaxation that varies from ca. 10 ps for the smallest particles examined (similar to4 nm diameter) to almost 400 ps for the 50 nm diameter particles. The relaxation times are proportional to the square of the radius, but do not depend on the initial temperature of. the particles (i.e., the pump laser power). For very small particles, the time scale for energy dissipation is comparable to the time scale for electron-phonon coupling, which implies that significant energy loss occurs before the electrons and phonons reach thermal equilibrium within the particle.