This paper discusses the effects of adding particulate filler to a system undergoing liquid-liquid thermally induced phase separation (L-LTIPS). While much is known about the growth of droplets in L-LTIPS, little is known about the effect particular fillers have on droplet growth and the final cell size in the resulting microporous membranes. In this work, zeolite particles are shown to have a significant effect on the final cell size of these microporous membranes, the extent of which depends on the particle loading and processing conditions used to form the membrane. Two polymer-diluent-zeolite systems are reported: isotactic polypropylene-diphenyl ether and poly(methyl methacrylate)-cyclohexanol, both with zeolite 4A particles. (c) 2008 Elsevier B.V. All rights reserved.