Membrane liquid loss of supported ionic liquid membranes (SILMs) under high cross-membrane pressure difference (Delta p) is an urgent issue in gas separation. In our research, it was found that the even serious loss of the membrane liquid might not lead to the degradation of SILMs. To explore the underlying membrane liquid loss mechanism of SILMs, SILMs were prepared with 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) immobilized in polyethersulfone (PES), nylon 6 (N6), and polyvinylidene fluoride (PVDF) microporous membranes. The membrane liquid loss behaviors of the SILMs were investigated in terms of gas separation performance and the weight loss of membrane liquid (ML loss). The PES SILM has the least ML loss owing to the less interconnected network, small maximum pore size of the membrane, as well as the great interaction of PES with [bmim][BF4]. Membrane compression was proposed as one of the main reasons of the membrane liquid loss of the SILMs, which did not lead to the degradation of the PES SILM because of the reduction of the pore diameter and the thickness of the membrane. A mathematical model was established to evaluate the dimensional changes of the PES SILM caused by compression. The membrane thickness is reduced and the pore size and porosity are also decreased, when Delta p is increased only from 0.1 to 0.3 MPa. (c) 2012 Elsevier B.V. All rights reserved.