The dynamic processes of photoreduction of chloroauric acid and subsequent formation of gold nanoparticles were directly observed by means of the single-shot, near-field heterodyne transient grating method. For a polyvinylpyrrolidone (PVP) concentration of less than 0.5 mM, the observed transient responses consisted of four exponential functions that corresponded to thermal decay and the reaction and diffusion processes of three chemical species in the photoreduction processes. Another exponential component was found for PVP concentrations above 0.5 mM, and it was proposed that this component corresponds to the diffusion of gold nanoparticles from the decay time dependence on the pump intensity. (C) 2008 Elsevier B.V. All rights reserved.