The effect of hygrothermal aging on proton exchange membrane (PEM) is a critical issue because the membrane is always subjected to high temperature and humidity under operation conditions. In this study, the mechanical responses of the Nafion 212 membrane subjected to hygrothermal aging are investigated experimentally through uniaxial tensile, stress relaxation and creep-recovery tests. Higher modulus and tensile strength are obtained for longer membrane aging because of the formation of crosslinks during the aging process. The creep strain is divided into three components to quantitatively assess the effect of hygrothermal aging on the mechanical responses of the membrane, and the predominant creep strain component changes as the hygrothermal aging time increases. In general, hygrothermal aging significantly affected the viscous flow strain and delayed elastic strain in correspondence to chain disentanglement and slippage, respectively. In addition, the effect of temperature on the creep behavior of the membrane aging for various times is also studied. The delayed elastic strain increases significantly as the aging time increases, while the instantaneous strain is least affected. (C) 2013 Elsevier B.V. All rights reserved.