Free-volume hole properties (free-volume size, fraction, and distribution) of polycarbonate (PC) exposed to CO2 and N-2 gases are investigated using positron annihilation lifetime (PAL) spectroscopy. In PC/CO2 systems, free-volume hole size and fraction significantly increase as a function of CO2 pressure in the range 0-1000 psi. Hysteresis is observed during CO2 sorption/desorption. In PC/N-2 systems, free-volume hole size and fraction reach maxima at a pressure of about 800 psi but no hysteresis is observed. Free-volume hole distribution in the CO2-exposed polymers is found to be significantly broader than in unexposed samples. The observed Variation in free-volume properties of PC exposed to gas is discussed in terms of penetrant plasticization, gas hydrostatic pressure effect, and molecular filing and creation of holes in the polymer matrix.