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The diffusion of polymer molecules across an interface is an important topic in polymer physics. Measuring this diffusion process requires a labeling experiment so that the mixing of otherwise identical molecules can be followed. Here we examine the use of nonradiative energy transfer to follow the interdiffusion of polymer molecules during the annealing of latex films. Those films are prepared from a mixture of phenanthrene (Phe, the donor)- and anthracene(An, the acceptor)-labeled poly(butyl methacrylate) (PBMA) latex particles. We examine in some detail the kinetics of direct energy transfer (DET) at an interface and the evolution of the DET kinetics as the components mix. Time resolved and steady state measurement approaches are compared, and the former is employed to examine molecular diffusion in a series PBMA latex films. Various diffusion models are considered. Diffusion coefficients are calculated by treating the data in terms of a Fickian spherical diffusion model. A film from one set of samples is similar in molecular weight to that studied by small angle neutron scattering (SANS) [K. Hahn, G. Ley, H. Schuller, and R. Oberthur, Colloid Polym. Sci. 264, 1029 (1986)]. Diffusion coefficients calculated from both sets of experiments using similar Fickian diffusion models are very similar in magnitude. [ABSTRACT FROM AUTHOR]