The porosity of Shirasu Porous Glass (SPG) membranes was controlled continuously with dextran produced by the enzymatic reaction of dextransucrase (DSase). DSase was immobilized inside the pores of an SPG membrane at a density of 1.5 U g(-1), and a sucrose solution was subsequently permeated through the DSase-immobilized SPG membrane to produce dextran from the active site of DSase. Varying the sucrose concentration from 5 to 25 mg L-1, resulted in a change in the rate of dextran produced, due to a decrease in the intermediate formation of the substrate-enzyme complex. Using the Kozeny-Carman equation, and based on the relationship between the pressure loss and pure water permeation rate, the membrane porosity could be varied between 38 and 66% by changing the amount of dextran produced. (c) 2007 Society of Chemical Industry