The phase behavior of blends of ABC triblock and ac diblock copolymers is examined using self-consistent mean-field theory. Several equilibrium lamellar structures are observed, depending on the volume fraction of the diblocks, <()over bar>(2), the monomer interactions, and the degrees of polymerization of the copolymers. For segregations just above the order-disorder transition the triblocks and diblocks mix together to form centrosymmetric lamellae. As the segregation is increased, the triblocks and diblocks spatially separate either by macrophase-separating or by forming a noncentrosymmetric (NCS) phase of alternating layers of triblock and diblock (...ABCcaABCca...). The, NCS phase is stable over a narrow region near <()over bar>(2) = 0.4. This region is widest near the critical point on the phase coexistence curve and narrows to terminate at a triple point at higher segregation. Above the triple point there is two-phase coexistence between almost pure triblock and diblock phases. The theoretical phase diagram is consistent with experiments.