INTRODUCTION: GAS SORPTION AND TRANSPORT PROPERTIES OF POLYMERS HAVE A SIGNIFICANT IMPACT ON THE MATERIAL PERFORMANCE, SUCH AS DURABILITY (OXYGEN DIFFUSION), LONG-TERM BEHAVIOUR, PLASTICIZATION AND DILATION OF POLYMERIC MATERIALS, AND THEIR USE IN ENVIRONMENTALLY IMPORTANT AND COMMERCIALLY AVIABLE APPLICATIONS OF MEMBRANE TECHNOLOGY. EXPERIMENTAL TECHNIQUES AS WELL AS THEORETICAL APPROACHES HAVE BEEN EMPLOYED TO ADDRESS THE CHANGES IN THE STRUCTURE (FREE VOLUME) AND DYNAMICS (GLASS TRANSITION, RELAXATION PROCESSES) OF POLYMERS UPON GAS SORPTION.MOLECULAR SIMULATION METHODS, ON THE OTHER HAND, CAN PRODUCE MOST INFORMATION AT THE ATOMISTIC LEVEL ABOUT THE SORPTION AND DIFFUSION PROCESSES.ALTHOUGH IN MOST OF THE REPORTS THE INVESTIGATORS HAVE DEALT WITH FLEXIBLE POLYMER CHAINS OF SIMPLE CHEMICAL STRUCTURE, SUCH AS POLYETHYLENE AND POLYPROPYLENE MORE RECENT WORKS ARE DEVOTED TO POLYMER-PENETRANT SYSTEMS OF MORE COMPLEX CHEMICAL STRUCTURES SUCH AS POLYIMIDES [1], POLYSTYRENE [2], POLY (ETHYLENE TEREPHTHALATE) [3], AND POLYAMIDE-6, 6 [4].