The sorption of N-2, Ar, and CO2 in the glassy polymer with intrinsic microporosity PIM-1 has been investigated across a broad temperature range, from cryogenic (77 K) to near room temperature (275 K). The data obtained have been interpreted by the non-equilibrium thermodynamic model for glassy polymers (NET-GP), which accurately describes the experimental behaviors, and is used to predict the solubility of the same gases above room temperature. The analysis reveals that the same mechanism drives the sorption process over the broad temperature range inspected, with physical dissolution of the gas in the glassy matrix accompanied by significant swelling of the polymer occurring largely in the central portion of the isotherm at cryogenic temperatures. These findings clearly support the idea that calculation of the BET surface area for amorphous high free-volume glassy polymers does not represent an actual real physical property and suggest that its value is an unsuitable metric of polymer material properties. The gas uptake obtained in the cryogenic sorption isotherm is clearly related to the excess free volume and may instead serve as a more appropriate metric for comparison between different polymers.