A recent analysis of the enthalpy-driven swelling of a polymer brush by a compatible homopolymer leads to a simple swelling criterion that delineates the entropic and the enthalpic contributions. We consider the implications of this model for the formation of microphase in binary blends of a symmetric diblock copolymer (A-b-B) for which each block has a degree of polymerization (DP) = N and a homopolymer (A, B or X) which has DP = P. The model suggests that, in A/A-b-B and B/A-b-B blends, microphase formation will occur only if NIP > 1. However, in X/A-b-B blends, where there is an exothermic interaction between X and A or B, the enthalpic swelling can enable microphase formation even if N/P < 1. We present morphological observations which are qualitatively consistent with these predictions.