This paper reports a deuterium effect involving poly(styrene-co-methyl methacrylate) alternating copolymer/poly(methyl methacrylate) homopolymer blends, AB/A, in which deuteration of the methyl methacrylate (MMA) monomer unit influences blend miscibility. Two blend compositions were investigated, 35%/65% and 50%/50%, using DSC and TEM. Perdeuteration of the MMA units of the copolymer promotes miscibility, perdeuteration of the MMA units of the homopolymer reduces miscibility, and the absence of perdeuteration exhibits intermediate miscibility. These results are interpreted using the concept that intramolecular repulsion within a copolymer can promote copolymer/homopolymer miscibility. Using the sequence-independent models for copolymer/homopolymer blends, we have determined chiPS,D-PMMA > chiPS,H-PMMA, which is consistent with measurements of these interaction parameters by others.