Polymer diffusion across interfaces at room temperature (21 degrees C) was analyzed by direct nonradiative energy transfer (DET) in labeled latex films. Two model latex polymers were examined : poly(butyl methacrylate) [PBMA, M-w, = 3.5 x 10(4), T-g (dry) = 21 degrees C] and a copolymer of 2-ethylhexyl methacrylate with 10 wt % (acetoacetoxy)ethyl methacrylate [P(EHMA-co-AAEM), M-w = 4.8 x 10(4), T-g (dry) = -7 degrees C]. Little energy transfer due to polymer diffusion was detected for the P(EHMA-co-AAEM) latex samples in the dispersed state or dried to solids content below ca. 90%, but above 90% solids, diffusion occurs among particles. For PBMA, diffusion occurs only after the film is dried (>97% solids) and aged. In the dry PBMA films, it requires 4-5 days at 21 degrees C to reach a significant extent of mixing (f(m) = 0.3-0.4). This corresponds to an estimated penetration depth d(app) of 30-40 nm and a mean apparent diffusion coefficient (D-app) of 5 x 10(-4) nm(2)/s. The corresponding D-app value for the dry P(EHMA-co-AAEM) sample is 5 x 10(-2) nm(2)/s, and it takes about 25-40 min for this polymer to reach f(m) of of 20-30 nm.