Two sequential urethane-acrylate interpenetrating network (IPN) systems were prepared in which the polyurethane (PU) phase is labeled with donor and acceptor dyes. Direct nonradiative energy transfer (DET) measurements on these systems indicate less efficient energy transfer in the IPN than in the corresponding pure PU matrix. This result is interpreted in terms of dilution of the dyes by mixing at the molecular level between the polyacrylate (PA) and PU components. Quantitative analysis of the changes in DET efficiency allows the extent of phase mixing to be calculated. The two SeqIPN’s, and PU(g15)(75%)PA(EHA)(PU100%)-P-100% and PU(g180)(100%)PA(IBA)(100%), were labeled by incorporating phenanthrene and anthracene diols into the reaction mixture. Fluorescence decays of phenanthrene in these samples were measured by the single-photon-timing technique and analyzed in terms of both the Forster model for DET and the Perrin model for static quenching. Both analyses gave similar extents of phase mixing, and these values are in good accord with the results of electron microscopy and dynamic mechanical experiments. Similar experiments were carried out during the polymerization reaction. These allowed features of the morphology evolution to be understood.