Interpenetrating polymer networks (IPNs) of polyurethane (PU) and polyacrylate (PA) were prepared to give very small domains (ca. 20 nm) of:PA dispersed in a PU matrix. In these samples the PU phase is labeled with phenanthrene, a donor for energy transfer experiments, and the PA phase is labeled with anthracene, the corresponding acceptor. Direct nonradiative energy transfer from phenanthrene to anthracene was used to study the IPN morphology. The interface and mixing between the microphases were analyzed in terms of a Helfand and Tagami (H-T) profile, which is commonly used to describe the interface in polymer blend systems. Exact fluorescence theoretical decay profiles were calculated according to the H-T profiles in a restricted geometry. Experimental fluorescence decays were compared to the theoretical decays, and the interface thickness was recovered. At the level of the model we used in our analysis, we found strong similarities between these IPNs and traditional polymer blends.