The intercelluar diffusion of poly(butyl methacrylate) (PBMA; glass transition temperature (T-g) = 21 degrees C) at 60 degrees C was monitored by fluorescence energy transfer in films formed from blends of PBMA latex and a high-T-g (hard) latex. Two types of hard latexes were used in blends : one composed of poly(methyl methacrylate) (PMMA) and the other having a high-T-g shell made of styrene and acrylic monomer units with a large central void (Rohm & Haas Ropaque, ROP). The diffusion rate of PBMA is significantly decreased in the presence of hard particles. The mean apparent diffusion coefficient (D-app) decreased from 0.3 to 0.2 nm(2)/s when 35 vol % ROP particles (400 Mn) are present in the film and to 0.1 nm(2)/s at 75 vol % ROP. The presence of similar-sized PMMA (400 nm) particles showed a slightly larger effect than ROP on the PBMA diffusion : D-app decreased by a factor of 2 at 35 vol % PMMA and a factor of 4 at 75 vol % PMMA. At a constant volume fraction of the hard particle additive, the PBMA diffusion rate decreases significantly when the diameter of PMMA particles is reduced from 400 to 67 nm. These effects can be explained in terms of a model in which a more rigid PBMA layer forms, with a slower diffusion rate, near the hard particle surface.