Sorption of CO2 in poly(methyl methacrylate) at 35-200 degrees C and concurrent dilation of the polymer at 35-85 degrees C over a pressure range up to 50 atm were studied. Dissolution and Flory-Huggins interaction parameters for the gas in the polymer, not only in the rubbery state but also in the glassy state? were estimated by analyzing the sorption data above the glass transition temperature (T-g0, 105 degrees C). Isothermal glass transition of the polymer/gas system was observed on isotherms of sorption and dilation below T-g0. Partial molar volumes of sorbed COB determined from the sorption and dilation isotherms increased with increasing concentration to the glass transition concentration. These isotherms were also analyzed on the basis of ex-tended dual-mode models of sorption and dilation. From obtained parameters of the dual-mode models, nonequilibrium properties such as mean size and number of microvoids for the pure polymer and the CO2-sorbed polymer in the glassy state were evaluated. The mean size, dependent upon CO2 exposure history of the polymer, was in the range of 20-100 A(3), and the number of microvoid ((1-18) x 10(20) voids/cm(3)) was dependent upon both temperature and the exposure history.