AbstractThe sorption of CO2and the noble gases Ne and Ar in semicrystalline glassy poly(butylene terephthalate) (PBTP) films was measured by the gravimetric method with a recording microbalance at 298 K. The sorption of CO2was found to be significantly higher than that of Ne and Ar. This is attributed to a specific interaction between CO2and PBTP. The sorption isotherm for CO2was analyzed by the dual‐mode sorption model, while the sorption behaviors for Ne and Ar did not follow this model. Their sorption isotherms can be described by the sorption model developed here for the noble gases in PBTP. A critical adsorption pressurep*that is dependent mainly on the relative size of the frozen microvoids and the noble gas atoms is defined in this model. The Langmuir adsorption for describing the sorption in this gas/polymer system in addition to Henry's solubility happens just above thisp*, whereas only Henry's solubility takes place belowp*. © 1993 John Wiley&Sons, I