The surface morphology and polymer-polymer interaction parameter (chi(12)) of UCST blend systems comprising isotactic polystyrene and poly(4-methyl styrene) (P4MS) were investigated using atomic-force microscopy (AFM) and differential scanning calorimetry (DSC). From the measured glass transition temperature and the specific heat increments (DeltaC(p)) at T-g, it was found that the P4MS dissolved more easily in the iPS rich-phase than did the iPS in the P4MS rich-phase. AFM result also supported that the compatibility increased more in the regions of P4MS-rich compositions than in the regions of PS-rich compositions of the PS/P4MS blends. From the measured T-g's and apparent weight fractions of iPS and P4MS dissolved in each phase, the values of the Flory-Huggins interaction parameter (chi(12)) were determined to be 0.0163-0.0232 depending on the composition. These results indicate that the chi(12) is quite dependend on the apparent volume fraction of the polymers dissolved in each phase. The values of chi(12) calculated from this work (method based on T-g's of phases) were lower than those estimated using an earlier method based on the UCST or clarity temperatures. All values of chi(12) are greater than the values of interaction parameter at the critical point (chi(12))c. This fact indicates that the iPS/P4MS blend are immiscible for all blend compositions. The surface of the phase-separated blend system was mostly covered with the P4MS rich-phase owing to its lower surface free energy in comparison with that of the neat iPS. The mechanism of surface phase separation for the P4MS blends with aPS or iPS is governed by two factors: (1) difference in the solubility of the two polymers in the solvent and (2) surface free energy. (C) 2003 Elsevier Science Ltd. All rights reserved.