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The enantioselective transport of DL-phenylalanine across a polymeric membrane plasticized with o-nitrophenyl octyl ether and containing a chiral crown ether was investigated. The transport behavior was dependent on the type of polymers. Poly (vinyl chloride) gave an excellent membrane keeping both fluxes and optical resolution ratio constant for a long time. A plasticized cellulose triacetate membrane initially showed a high optical resolution ratio, which was decreased rapidly and approached a steady value of about 7. This value was approximately equal to those obtained with poly (vinyl chloride) membranes and supported liquid membranes. On the other hand, a plasticized cellulose nitrate membrane gave a small optical resolution ratio of less than 4 after an initial rapid decrease. The experimental results obtained with membranes of various thicknesses showed that the rate-determining step of the transport was the diffusion in the membranes.