New experimental results for the solubility of nitrogen and carbon dioxide in polystyrene are reported, accompanied by data on the change in volume of the polymer caused by the sorption process. The two phenomena were measured simultaneously with a combined technique, in which the quantity of penetrating fluid introduced into the system was evaluated by pressure-decay measurements in a calibrated volume, whereas a vibrating-wire force sensor was employed for weighing the polymer sample during sorption inside of the high-pressure equilibrium cell. The use of the two techniques was necessary because the effects of Swelling and solubility could not be decoupled in a single gravimetric or pressure-decay measurement. The sorption of nitrogen in polystyrene was studied along three isotherms from 313 to 353 K at pressures up to 70 MPa. The sorption of carbon dioxide was measured along four isotherms from 338 to 402 K up to 45 MPa. The results are compared with values from the literature when possible, although our data extend significantly the pressure ranges of the latter. The uncertainties affecting our measurements with nitrogen are I mg of N-2/g of polystyrene in solubility and 0.1% of the volume of the polymer. For carbon dioxide, the uncertainties are 5 mg of N-2/g of polystyrene and 0.5% respectively, carbon dioxide being about 1 order of magnitude more soluble than nitrogen.