Dextran curves are commonly used to measure the sieving characteristics of UF membranes. The use of refractive index limits the sensitivity of the dextran test, making it difficult to accurately measure sieving lower than 0.01. However, most UF processes require the product sieving to be less than 0.001 to avoid yield losses. Thus there exists a gap between dextran characterization of UF membranes and UF process requirement. In this work a new method called the Flex test has been developed which uses neutral and charged fluorescent dextrans to characterize both neutral and charged UF membranes. Data demonstrating the utility of this method have been generated with 100, 300 and 1000 kDa, neutral and charged composite regenerated cellulose membranes. It has been shown that pore size of neutral membranes can be characterized using neutral fluorescent dextrans to accurately measure sieving as low as 0.001. On the other hand, both pore size and charge of charged membranes can be characterized using neutral dextrans and charged fluorescent dextrans. Neutral dextrans capture the effect of membrane pore size alone on sieving, and charged fluorescent dextrans capture the combined effect of membrane pore size and charge on sieving. This characterization of both membrane pore size and charge is important for all charged membrane applications. A tool called the Flex test tables has also been proposed to succinctly transfer this characterization information from membrane manufacturer to membrane user. This characterization method is valuable for all UF and HPTFF applications, where membrane pore size and charge affect separation performance. (C) 2004 Elsevier B.V. All rights reserved.