In the present work, the effect of adding nonsolvent in the casting solution on the porosity of asymmetric TPX (poly(4-methyl-1-pentene)) membranes was systematically investigated. A series of alcohols, with carbon number ranging from 2 to 14, was added in the casting solution (TPX/cyclohexane) to alter the porosity of two types of asymmetric TPX membranes, prepared by using ethanol and 1-propanol as the coagulation medium. It was found that the effect of nonsolvent on membrane porosity is different for the two types of membranes and the difference can be reasoned by considering the exchange rate between the polymer solvent and the coagulation medium during membrane formation. The results indicate that, for the membrane formation system with low exchange rate between coagulant and solvent, the membrane porosity is controlled by the coagulation value, defined as the volume of coagulant required to demix the casting solution. On the other hand, for the system with high exchange rate, the membrane porosity is not controlled by the coagulation value but by the penetration speed of the coagulant front moving through the casting solution.