Small angle neutron scattering is used to study the phase behavior of mixtures of two immiscible homopolymers (A and B) and a diblock copolymer (A-C) wherein B and C chains exhibit attractive interactions (negative Flory-Huggins interaction parameter) and the other pairs of chains exhibit repulsive interactions. This study explores the effect of homopolymer molecular weight asymmetry (N-A/N-B not equal I, where N-I is the number of monomer units per chain in homopolymer I) at fixed segregation strength of the homopolymers. The temperature windows over which lamellae, microemulsions, macrophase separation, and homogeneous phases are found are affected qualitatively by N-A/N-B. In particular, a homogeneous window that was not present in symmetric A/B/A-C blends is seen when N-A/N-B exceeds a critical value.