It has long been recognized that the interphase, i.e., the region between adjacent phases in heterogeneous materials, is a major factor in determining many of the mechanical, optical, and electronic properties of such systems. Interphases are, in general, not abrupt, but exhibit a gradient in the composition from one pure phase to the other. For block copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA), it has been shown, using neutron reflectivity (NR) and fluorescence measurements, that the interphase can extend over several (similar to 5) nanometers. In this paper, it is shown for block copolymers of perdeuterated polystyrene and poly(methyl methacrylate) (P(d-S-b-MMA)) that proton-deuterium CP/MAS experiments can be used for spin counting of deuterons in the interphase. Our approach takes into account the varying compositional profile across the interphase. The results are in reasonable agreement with those derived from NR studies. The basic assumptions underlying our approach to deuteron counting at the interphase are discussed.