The effect of gas pressure on membrane aging was studied thoroughly by applying feed gas pressure continuously or by reducing the feed gas pressure intermittently. Resemblance of aging curves of gas separation membranes to visco-elastic response curves was observed for all the studied membranes. Membrane aging curves consist of elastic and viscous components. It was further concluded that the elastic component of polymer relaxation was enhanced when membranes were aged at lower pressures while the viscous component was enhanced at higher gas pressures. The transition from the elasticity dominant to the viscosity dominant region occurred more quickly for thicker membranes.