Equilibrium sorption of n-butane and acetaldehyde in melt-processed poly(ethylene terephthalate) (PET) microtomed flakes is reported. The n-butane sorption isotherm at 35 degreesC is well described by the dual-mode model with the following parameters: k(D) = 0.017 cm(3) (STP)/ (cm3 amorphous polymer cmHg), C ' (H) = 1.3 cm(3) (STP) (cm3 amorphous polymer) and b = 0.029 cmHg(-1). Acetaldehyde isotherms at 35 and 45 degreesC may be described by the Flory-Huggins sorption model, suggesting that penetrant uptake in the non-equilibrium excess volume associated with the glassy polymer made a negligible contribution to the overall sorption level at the conditions of this study. The heat of sorption was essentially equal to the enthalpy of condensation of pure acetaldehyde. At 45 degreesC and acetaldehyde pressures at or above 40.0 cmHg, acetaldehyde triggers significant crystallization of PET (up to 37 wt%) with increasing concentration. Subsequent sorption experiments at very low penetrant activity reveal solubility coefficients that are markedly higher in the penetrant-crystallized sample than in the initially highly amorphous sample. This result suggests the acetaldehyde-induced formation of microvoids (which act as highly efficient penetrant sorption sites) in the polymer sample. Based on these and Literature data, the logarithm of infinite dilution penetrant solubility in amorphous regions of PET was well-correlated with penetrant condensability as characterized by T-c, penetrant critical temperature, or by (T-c/T)(2) where T is the experiment temperature. Infinite dilution, amorphous phase penetrant diffusion coefficients in PET decreased according to a power law relation with increasing penetrant critical volume.