Miscibility behavior of blends of isotactic and syndiotactic poly(methyl methacrylate) (iPMMA and sPMMA) was investigated as a function of annealing temperature and molecular weight of the iPMMA component. Annealing experiments clearly indicate that for the high molecular weight iPMMA sample formation of stereocomplexes between isotactic and syndiotactic PMMAs is hindered. Annealing experiments carried out in the temperature range 313-473 K, followed by DSC measurements, showed that iPMMA is completely miscible with sPMMA only in a narrow temperature range around 350 K. To explain these experimental data, a simple theoretical model for the prediction of phase behavior of polymer blends involving dispersive interactions, is presented. This model accounts for subtle structural differences (e.g., stereoisomerism) between the repeat units of the blend components, and by incorporating the surface to volume ratios into the original Flory-Muggins solubility parameter approach, which makes it possible to account for packing effects on blend miscibility. The model is able to predict the presence of both UCST and LCST experimentally observed for stereoregular PMMA blends, without employing an equation of state approach. © 2007 American Chemical Society.