The effect of adding a second diluent on the membrane formation via thermally induced phase separation was investigated both by dissipative particle dynamics (DPD) simulation and experiment, taking into account the polypropylene/oleic acid system with neutral substrate on mesoscale. The simulations of the effect of primary/second diluent ratio on the membrane formation proved that adding a second diluent (poor diluent of the polymer) to a polymer/primary diluent (good diluent of the polymer) system would induce an L-L phase separation in which an S-L phase separation would occur. Moreover, the interaction parameter between the primary diluent and the second diluent had a significant influence on the phase separation and should be considered as a main factor for the second diluent selection. To verify the results obtained by simulations, experiments were conducted and the experiment conclusions were consistent with the simulation results. The information obtained from this study benefits to understand the membrane formation of polymer/mixed diluent system. (C) 2012 Elsevier B.V. All rights reserved.