In this contribution are investigated sorption thermodynamics and mass transport properties of propane and methanol in totally amorphous poly-(2,6- dimethyl-1,4-phenylene)oxide (aPPO) and semi-crystalline PPO (bPPO) samples. Semi-crystalline PPO, obtained by treating an amorphous sample with benzene vapours, is characterized by a crystalline phase endowed with empty spaces. In contrast to what is commonly observed in semi-crystalline polymers, bPPO shows a larger sorption capacity and faster sorption kinetics of low molecular weight compounds as compared to the totally amorphous sample: this behaviour, observed also in the case of sorption of liquid methanol, is discussed in terms of the morphology of the samples, attributing the observed peculiarities prevalently to the non negligible sorption capacity of crystalline domains associated to the presence of empty space in the crystalline structure. This structural aspect is supported by DSC analysis, which points out that the crystalline domains display a very low melting temperature. Sorption enthalpies of propane in aPPO and bPPO were also determined, on the basis of experimental sorption isotherms measured at different temperatures and were discussed in terms of the involvement of crystalline domains in the sorption process. Taken as a whole, sorption experiments indicate that semi-crystalline PPO is a promising candidate for use as membrane material in gas separation processes, in view of the remarkably high values of solubility and diffusivity. (C) 2013 Elsevier B.V. All rights reserved.