In this paper, we examine the influence of silica as fillers on polymer interdiffusion in poly(butyl methacrylate) latex films. We carry out fluorescence resonance energy transfer (FRET) measurements on latex films that allow us to follow the extent of polymer diffusion as a function of time after the latex/pigment dispersion dries. In this study, we compare four different types of colloidal silica and discuss how fillers affect the rate of polymer interdiffusion in latex films. The efficiency of energy transfer data for newly formed films indicate that 54 and 73 nm diameter SiO2 particles have little or no effect on the interfacial area. between donor- and acceptor-labeled latex cells, whereas the 16 and 27 nm SiO2 particles significantly reduce the interfacial area with increasing amounts of filler. The maximum efficiency of energy transfer data indicate that 16 and 27 nm SiO2 also affect the extent of mixing that can be achieved in 200 h annealing at 60 degreesC. The rate of polymer interdiffusion in latex films is retarded as one increases the amount of filler, and this rate decreases as the filler size decreases. Our results can be explained with a free volume model that assumes that the surface of the silica particles raises the effective glass transition temperature of the matrix.