EVAL copolymer was precipitated from water and octanol to form, respectively, asymmetric and particulate membranes in an isothermal immersion precipitation process. Permeability of these membranes was examined with respect to dextran samples of various sizes. The results indicate that asymmetric membranes reject large dextran molecules and let through small molecules for the pressure range of 0.25-0.75 kg/cm(2). The particulate membranes, however, exhibit an unusual filtration behaviour; i.e. there exists a maximum rejection at intermediate dextran molecular weight. This implies that small molecules tend to be trapped inside the nano-pores within the EVAL particles whereas large molecules travel through the tortuous channels outside the particles during a filtration operation.