An extraordinary diffusional behaviour of binary solvent mixtures in a polyacrylonitrile membrane was investigated by both permeation experiment and numerical simulation. The component of binary mixture which diffuses very slowly in the case of single component diffuses several times as fast as that expected from the diffusion kinetics for a single component solvent. A site exchanging model was assumed in which the slow component exchanges the neighboring site with another fast component and this exchange increases the diffusion effectively. Simulation by use of this model was examined and a good agreement with the permeation experiments was obtained semiquantitatively. The present simulation may provide a good estimation for evaluating the cooperative membrane permeation for mixed solvents.