The phase diagram of symmetric ternary blends of diblock copolymers and homopolymers in thin films was determined as a function of increasing volume fraction of homopolymer (phi(H)) and was similar to that for these materials in the bulk. Blends with compositions in the lamellar region of the diagram (phi(H)< or =0.4) could be directed to assemble into ordered lamellar arrays on chemically striped surfaces if the characteristic blend dimension (L(B)) and the period of the stripes (L(S)) were commensurate such that L(S)=L(B)+/-0.10L(B). Blends with compositions in the microemulsion region of the diagram (phi(H) approximately 0.6) assembled into defect-free lamellar phases on patterned surfaces with L(S)> or =L(B), but formed coexisting lamellar (with period L(S)) and homopolymer-rich phases when L(S)<L(B).