The structure of the macro-porous gel-filled (MPG) ultrafiltration membranes used in this study consist of a porous support matrix filled with an 'active polymer network' which allows for dynamic control of the effective pore size of the membrane through appropriate adjustment of the solution pH and/or ionic strength. The performance of MPG membranes was shown to be significantly different from commercially available thin-film composite (TFC) ultrafiltration membranes. With MPG membranes, only charge based rejection was observed to affect the sieving behavior of dextran solutes. In this study MPG membranes made from three different porous supports were extensively characterized. Flux measurements at various solution pH and salt concentrations were used to quantify the 'valve effect' of the MPG membranes and indicated that the porosity of the support matrix had a significant effect on the membrane performance. The transmissions of neutral and charged dextrans were also found to be dependent on the support matrix. (C) 2009 Elsevier B.V. All rights reserved.