Using optical and scanning electron microscopies, dynamic mechanical analysis, and differential scanning calorimetry, this study investigated effects of tacticity in PMMAs on the phase behavior in three pairs of binary blends comprising any two of isotactic-, syndiotactic-, and atactic-PMMAs. Miscibility was confirmed for all three binary blends (i.e., aPMMA/iPMMA, aPMMA/sPMMA, and iPMMA/sPMMA); however, the T_gvs. composition relationships for the aPMMA/iPMMA blend differ significantly. Configurational tacticity may lead to subtle differences in molecular interactions in the tactic polymers; and in turn such difference in the interactions causes variation in the glass transition and phase behavior in these three blends. This study also points out that the miscibility nature in iPMMA/sPMMA may constitute a prerequisite condition (necessary but not sufficient) for its tendency in forming complexes.