We present a theoretical study of the lamellar microphase of block copolymer/selective solvent blends in which the solvent is good for one block and either good or near-theta for the other. The calculations used numerical solutions to the equations of the self-consistent mean-field theory of polymer blends. Two types of systems were studied. In the first, all pure-component densities were taken to be equal, as were the two Kuhn statistical lengths, and the solvent was idealized as being a thermal for one of the blocks. In the other type, realistic values for these parameters were used; calculations were carried out for the particular case of PS-b-PBD/styrene. We examined the dependence of the lamellar thickness on molecular weight, overall concentration, and Flory interaction parameters, and we discuss the solvent and polymer density profiles. We compare the predictions for blends with selective solvents with previous ones for nonselective solvent cases, and we compare the idealized systems with real systems. The theory predicts some significant differences between the idealized and real systems.