The phase behavior and rheology of binary blends of polystyrene (PS) and poly(alpha-methylstyrene) (P alpha MS), exhibiting upper critical solution temperature (UCST), and binary blends of PS and poly(vinyl methyl ether) (PVME), exhibiting lower critical solution temperature (LCST), were investigated. For the study, (i) PS-40/P alpha MS-18, (ii) PS-38/P alpha MS-39, (iii) PS-40/P alpha MS-48, and (iv) PS-110/PVME-95 blend systems were prepared by solution casting. The results of differential scanning calorimetry suggest that each blend system investigated is miscible over the entire blend composition as evidenced by the single composition-dependent glass transition temperature. However, from oscillatory shear rheometry we observed evidence suggesting that microheterogeneity is present in the miscible region, as determined by cloud point measurements, at temperatures as far away as approximately 70 degrees C above the UCST of the PS/P alpha MS blend system and at temperatures as far away as only approximately 7 degrees C below the LCST of the PS/PVME blend system. Such observation leads us to conclude that the extent of dynamical composition fluctuations near the critical point depends on the chemical structures of a polymer pair. The observed difference in the extent of dynamical composition fluctuations between PS/P alpha MS and PS/PVME blend systems is interpreted by the difference in the temperature coefficient of the interaction parameter between the PS/P alpha MS and PS/PVME blend systems.