The effect of diluents on polymer crystallization and membrane morphology via thermally induced phase separation(TIPS) were studied by changing the composition of the mixed-diluents systematically, in the system of poly(4-methyl-1-pentene) (TPX)/dibutyl-phthalate (DBP)/di-n-octyl-phthalate (D-n-OP) with TPX concentration of 30 wt %. The TPX crystallization was observed with differential scanning calorimetry (DSC) and wide angle Xray diffraction (WAXD). The membranes were characterized with scanning electron microscopy (SEM), porosity, and pore size measurement. As the content of D-n-OP increased in n-mixed-diluents, the solubility with TPX increased, inducing the phase separation changing from liquid-liquid phase separation into solid-liquid phase separation, which changed the membrane morphology and structure. When the ratios of DBP to D-n-OP were 10 : 0, 7: 3; 5 : 5, and 3: 7, membranes were formed with cellular structure and well connected pores, while the ratio was 0 : 10, discernable spherulities were found with not well-formed pore structure. The effect of composition of the mixed-diluents on membrane morphology was more remarkable in TPX/dioctyl-sebacate (DOS)/dimethyl-phthalate (DMP) system, since good cellular structure was formed when the ratios of DOS to DMP were 10 : 0, 7 : 3, while spherulites were observed when 5: 5. Dual endotherm peaks behavior on DSC melting curves emerged for all the samples in this study, which was attributed to the special polymer crystallization behavior, primary crystallization, and secondary crystallization occurred when quenching the samples. As the content of D-n-OP increased, the secondary crystallization enhanced which induced the first endotherm peak on DSC melting curves moving to a lower temperature and the broadening of the overall melting peak, as well as the increasing of the overall crystallinity. (c) 2008 Wiley Periodicals, Inc.