We have studied the lamellar structure of binary A-b-B/A and ternary A-b-B/A/B copolymer/homopolymer blends near the microphase separation transition. The approach combines perturbative solutions to the modified diffusion equation with a model for the total A and B polymer density profiles. We calculated, approximately, the equilibrium domain and subdomain thicknesses, swelling of the copolymers by the homopolymers, individual polymer density profiles, and their dependence on the copolymer and homopolymer degrees of polymerization, composition, and Flory interaction parameter. The results were compared with numerical solutions to the full self-consistent theory for related copolymer/solvent blends and to three sets of experiments on copolymer/homopolymer blends. The calculations were consistent with the picture that added homopolymers tend to penetrate within the copolymers and swell them laterally and that the degree to which this occurs depends on the relative molecular weights of the copolymers and homopolymers. The tendency of added homopolymers to cause an increase or decrease in the domain thickness correlates with their tendency to stabilize or destabilize the microphase.