Positron lifetime measurements at room temperature have been carried out to determine the average interchain volume in a series of polycarbonates. Applying different hydrostatic pressures of Ar and CO2 and dissolving the corresponding molecules in the polymer allows one to determine the change of the nonoccupied holes of the free-volume by positronium (Ps). A semiempirical relation between the average Ps lifetime and the average free-volume size is used to determine quantitatively the fraction of free volume. Assuming a distribution of hole volumes and a preferential occupancy of large holes by the dissolved molecules gives rise to a decrease of the Ps lifetime in agreement with experiments at low pressures. At higher concentrations, swelling of the polymer occurs and the microstructure changes irreversibly leading to a larger free-volume and therefore to an increase of the Ps lifetime. Three different approaches are used in order to calculate the number density of holes. The results favor an uniform occupancy of all holes by Ps instead of a preferential occupancy of large holes. Depending on the chosen distribution the hole density varies between 2.3 x 10(21) and 6.6 x 10(21) cm(-3).