The effect of the microstructure on the phase behavior of mixtures of polybutadiene (PB) and poly(ethylene-co-l-butene) (FEB) with polystyrene (PS) has been investigated. A series of PBs with 1,2-addition content ranging from 7 to 93% were synthesized by anionic polymerization, and a portion of each was subsequently hydrogenated to yield PEB. Polymer pairs with blend compositions from 10 to 90 wt % were cast from toluene for each of the 16 PS/PB pairs and 7 PS/PEB pairs. Laser light : scattering was used to obtain cloud point measurements, which were then used to construct phase diagrams. It was found that, for constituent components with equivalent degrees of polymerization, PS/PEB pairs give rise to higher upper critical solution temperatures than PS/PB pairs, indicating that PS/PEB pairs are less miscible than PS/PB pairs. Experimental phase diagrams were curve-fitted to theoretical phase diagrams predicted From the Flory-Huggins theory with the expression for the interaction parameter alpha : alpha = alpha + b/T + c phi(PS)/T, where alpha is related to the Flory-Huggins interaction parameter chi by chi = alpha V-r, where V-r is the molar reference volume, T is the absolute temperature, and phi(PS) is the volume fraction of PS in the mixture. a values for PS/PB mixtures increase with increasing 1,2-addition (miscibility decreases) while a values for PS/PEB mixtures decrease (miscibility increases) with increasing l-butene content. Using these a values, the Helfand-Wasserman theory was applied to predict the order-disorder transition temperatures of PS-bloch-PB and PS-block-PEB copolymers with varying 1,2-addition and l-butene content, respectively.