The rejection of various molecular weight dextrans for two commercially available regenerated cellulose and one polyethersulfone membrane has been investigated. Experimentally determined rejection curves were compared to calculated rejection curves based on field-emission scanning electron microscopy imaging assuming that the membrane pore size distribution may be described by the log-normal distribution function. Results show relatively good agreement between calculated and experimental curves; however, the calculated curves generally predict a broader rejection curve. At lower rejection values, the actual rejection is less than calculated. Results also indicate that field-emission scanning electron microscopy, a routinely used imaging technique, may be used to estimate the rejection curve for a membrane, thus enabling more informed selection of an appropriate membrane for a given separation. The results of this work are relevant to ultrafiltration, which is used in the biotechnology industry for protein concentration and buffer exchange. The success of a given UF operation depends on efficient passage or rejection of the desired solute species in solution. (C) 2009 Elsevier B.V. All rights reserved.