The sorption behavior of small molecules like ethane and ethylene in poly (lactic acid) (PLA) was studied in the temperature interval from 283 to 313 K using a Quartz Crystal Microbalance (QCM). The effect of the polymer structure on the solubility selectivity of PLA films with respect to these two gases was studied using polymer with two different L:D ratios (98:2 and 80:20). Furthermore, the polymer films were submitted to different thermal treatments to address the influence of crystallinity and morphology of the noncrystalline fraction on the sorption behavior. The sorption results obtained indicate that ethylene solubility coefficient in annealed PJA 98:2 is about 26% higher than that of ethane and 41% higher in PLA 98:2 melted. The dual-mode sorption model describes well the sorption isotherms behavior, which is concave concerning the pressure axis. The fully amorphous PLA presents the better selectivity for the studied gases, since the crystallinity seems to produce a negative effect on the selectivity. (C) 2008 Wiley Periodicals, Inc.