The phase structure development in the surface of poly(methyl methacrylate)/polystyrene (PMMA/PS) blend thin film in the presence of supercritical carbon dioxide (SCCO2) was studied by means of phase contrast microscopy and atomic force microscopy. The surface structures obtained after spin-coating are far from thermodynamic equilibrium. A stable surface structure, which the PS-rich phase is elevated by PMMA-rich phase, is formed after annealed in supercritical CO2. The structure can be explained by the different aggregation of the PMMA and PS in the two interfaces: air-polymer and substrate-polymer. The different surface structures are investigated as a function of treatment pressure and temperature of supercritical CO2. A critical phase structure transition temperature/pressure was found at an explicit pressure/temperature by adjusting the condition of the supercritical CO2. The phase structure transition point indicates a glass transition point could be found by tuning the working condition Of SCCO2. Due to the experimental difficulties in the measurement of T-g of the thin films in supercritical CO2, this study, by observing the resulting surface structure formed in supercritical CO2, provides us a new angle to look at the problem. (C) 2002 Elsevier Science B.V. All rights reserved.