This paper examines the use of direct nonradiative energy transfer (DET) experiments to determine the interface thickness in films of symmetric poly(styrene-b-methyl methacrylate) (PS-PMMA) block copolymers. The films were prepared from mixtures of two polymers of identical length and compositions, labeled at their junctions with either a 9-phenanthryl or 2-anthryl group. The fluorescence decays were analyzed in terms of a model that takes account of the Helfand-Taganu distribution profile of polymer segments at the interface. Reliable data analysis required careful subtraction of a background fluorescence from the measured decay profiles. Unlike neutron reflectivity experiments. DET measurements are insensitive to capillary waves at the interface. This advantage is offset by a strong sensitivity to the characteristic distance for energy transfer R-0, which has to be determined separately. When we take R-0 = 2.3 nm, we recover an interface thickness of 4.8 nm. We obtain a theoretical value of 4.2 nm when using the Semenov's finite-chain correction to Helfand-Tagami prediction and the Flory-Huggins chi (FH) parameter recovered by Russell for partially deuterated d-PS-PMMA copolymer. When we employ the Callaghan and Paul chi (FH) value for undeuterated PS + PMMA blends, we obtain delta = 4.9 nm.