We use polymer random phase approximation (RPA) theory to calculate the microphase separation transition (MST) spinodal for an AB + C diblock copolymer-homopolymer blend where the C homopolymers are strongly attracted to the A segment of the copolymers. Our calculations indicate that one can shift the MST spinodal value of the A-B segmental interaction parameter (N-chi AB)(S) to significantly lower values [i.e., (N-chi AB)(S) < 10.5] upon the addition of a selectively attractive C homopolymer. For a sufficiently attractive C homopolymer, (N-chi AB)(S) can be pushed to negative values, indicating microphase separation in what would appear to be a completely miscible diblock copolymer. Furthermore, we show that microphase separation can occur in diblock copolymer-homopolymer blends where the segmental interactions between all polymer constituents are attractive. By tuning the value of (N-chi AB)(S) with a homopolymer additive, one is therefore able to tune the effective copolymer segregation strength and thus dramatically affect the blend phase behavior. (C) 2009 Wiley Periodicals, Inc. (dagger) J Polym Sci Part B: Polym Phys 47: 2083-2090, 2009