The microphase separation transition (MST) behavior has been characterized by small-angle X-ray scattering analyses for a series of binary blends of a poly(styrene-block-hydrogenated butadiene) diblock copolymer with either polystyrene or poly(hydrogenated butadiene) homopolymers. This copolymer is symmetric in many respects; however, the statistical segment lengths of the two components differ markedly. The solubility behavior is asymmetric with respect to homopolymer type, showing significantly lower solubility limits for blends with the latter homopolymer. Experimentally determined apparent spinodal temperatures and correlation lengths exhibit trends with homopolymer molecular weight and content that correspond qualitatively to theoretical predictions derived for perfectly symmetric copolymers. That is, there is a critical homopolymer molecular weight at which the addition of homopolymer has no effect on either the microphase separation transition temperature or the correlation length. For homopolymer molecular weights below this threshold, the MST temperature is depressed, while for higher homopolymer molecular weights, the MST temperature is elevated and the correlation length increases with homopolymer addition. The experimental critical homopolymer molecular weight is found to be substantially lower than the predicted value of N/4, where N is the total copolymer molecular weight. The experimental correlation lengths depend approximately linearly on homopolymer composition and molecular weight, as predicted, but the magnitude of the molecular weight dependence is underestimated, leading to a mismatch in the theoretical and experimental correlation lengths. When the theoretical correlation length is adjusted empirically so as to match the experimental value, the entire scattering profile compares well with the predictions for a perfectly symmetric block copolymer. These results provide estimates of the segmental interaction parameters that agree closely with values obtained previously for the neat block copolymer. © 1989, American Chemical Society. All rights reserved.