Steady state fluorescence (SSF) technique was used to study the void closure process during coalescence of hard latex particles. Latex films were prepared by annealing pyrene (P-y)-labelled poly(methyl methacrylate) particles above the glass transition temperature. During the annealing processes, the optical clarity of the film increased considerably. Direct fluorescence emission from excited pyrene was monitored as a function of annealing temperature to detect these changes. Scanning electron microscopy in conjunction with Monte Carlo simulations of photon diffusion in latex film were used to interpret the fluorescence results. Void Closure temperature (T-c) and time (t(c)) were measured at the point at which the fluorescence emission intensity becomes maximum. This was associated with the longest optical path of a photon in latex film. (C) 1997 John Wiley & Sons, Inc.