Porous polydimethylsiloxane (PDMS) membranes were prepared by catalytic hydrosilyiation cure of a silicone compound containing a vinyl group and a SiH group in the presence of additives such as water, ethanol, ethylene glycol, and 2-methoxyethanol. The pores were formed in the membrane as a result of hydrogen foams produced by the reaction between the SiH group and the OH group of the additive during the cure. The cure reaction, hydrogen generation, and properties of the porous membranes that resulted were examined as a function of the additive concentration in the compound at various cure temperatures. As application of the membrane, the ultrafiltration properties were determined. © 1993 John Wiley & Sons, Inc.