We present solubility values of gas mixtures (CH4 + CO2 and CH4 + H-2) in polyethylene below its melting temperature. The objective of such studies is to assess the existence of specific interactions between the matrix and one type of penetrant molecule. Data were acquired using both experiments and molecular simulations. Experiments were performed on a medium density polyethylene with pure and mixed gases for temperatures in the range 308-313 K. Hence, the solubility coefficient of each component of a gas mixture was determined. Monte Carlo simulations in the osmotic ensemble were also used to predict gas concentrations in the polymer phase. A good agreement with experimental data is observed. This is a significant validation of the use of an ad hoc constraint in the osmotic ensemble simulations to mimic the overall effect of the crystalline regions and to predict quantitatively solubility data in this semicrystalline system. (C) 2011 Elsevier B. V. All rights reserved.