Abstract: We have studied the effect of the plasticizer dioctyl phthalate (DOP) on oxygen-enrichment performance of polyvinyl chloride(PVC). The permeability of oxygen of polyvinyl chloride increases pronouncedly with the content of DOP and approaches to an asymptotic value 2×10^-9cm³(STP)·cm/cm·s·cmHg,two orders higher than that of PVC without plasticizers. The separatied factor α_02/N2 decreases with the increase of the content of DOP but higher than or equals to that of PVC itself. The higher the content of DOP,the lower the glass transition temperature and hence the greater the fractional free volume of PVC.The significant effect of DOP on the permeability of oxygen could be, therefore,quite satisfactorily explained with the free volume theory of diffusion in polymers.