Immiscible polymer blend films were formed by air drying aqueous dispersions containing mixtures of a high-T-g latex, poly(methyl methacrylate), and a film-forming low-T-g latex, poly(butyl methacrylate-co-butyl acrylate). Fluorescence energy transfer experiments were used to characterize the interfaces in these films, in which one component was labeled with a donor dye and the other with an acceptor. The quantum efficiency of energy transfer (Phi(ET)) between the donors and accepters is influenced by the interfacial contact area between the two polymer phases. As the amount of soft component in the blend is increased, Phi(ET) approaches an asymptotic value, consistent with complete coverage of the hard polymer surface with soft polymer. This limiting extent of energy transfer is very sensitive to the total surface area in the film, with correspondingly more energy transfer at constant volume fraction for small hard particles. Some of the details of the energy transfer are revealed through a fluorescence lifetime distribution analysis. The presence of ionic surfactant (sodium dodecyl sulfate) in the dispersion from which the latex blend film is prepared reduces the cross-boundary energy transfer by 30%, which implies that in these films the surfactant decreases the interfacial contact. After annealing the surfactant-free blends above 100 degrees C, we observe an increase in energy transfer, consistent with a broader interface between the two polymers.