Composition dependence of phase behavior in the mixture system of polystyrene (PS) with poly(cyclohexthyl methacrylate) (PCHMA) was examined in details using differential scanning calorimetry, optical and scanning electron microscopy, and infrared spectroscopy. This study shows that the ＇transparent＇ blends of PS/PCHMA with an apparent single T-g, having been thought to be miscible in earlier reports, actually displayed micro-heterogeneity domains of about 100-300 Angstrom. It is thus not appropriate to assess miscibility to the optically clear blends of PS/PCHMA; however, partial miscibility does exist. The PCHMA-rich (PCHMA >70 wt%) compositions are indeed miscible but the PS-rich compositions are only partially miscible with sub-micron discrete phase domains beyond detection limit of optical microscopy. The proximity of the T(g)s also prevented that the thermal transition of the micro phase domains in the PS-rich blends being resolved. The criterion of single T-g commonly employed in in assessing phase behavior should be used with extra precaution in cases where microheterogeneity exists, whose ambiguity is even compounded in cases where the T(g)s of the constituent polymers are close. The FT-ir result indicates that intermolecular interactions are weak or non-specific for the PCHMA-PS pair.