Abstract The coagulation of ethylene vinyl alcohol (EVAL) copolymer membranes was investigated with a solvent/nonsolvent mixture. A two step mechanism model is presented which describes mass transfer processes occurring for specified membrane formation systems. The various morphology structures can be reasonably explained by this model by changing the solvent content of the coagulation bath. It clearly shows that the membrane structure actually depends on the local composition at phase transition and is controlled mainly by kinetic and structural factors rather than thermodynamic equilibrium. This model also points out some aspects neglected of membrane formation in published research. This analysis provides better understanding and correct control of membrane formation techniques.