Asymmetric ceramic-polymer membranes were prepared by dry and wet phase inversion processes using dispersions containing cellulose acetate, acetone, water and colloidal alumina particles. When the alumina content is low, the microstructure contains macrovoids (pore diameter similar to 15 mu m) below a dense polymer rich skin. The addition of greater amounts of alumina results in a microstructural transition from macrovoids to a uniform small pore network (pore diameter similar to 2 mu m). However, the alumina particles have a negligible effect on the porosity of the dense skin that forms at the air/membrane or water immersion bath/membrane interface. In all membranes, the alumina particles are unagglomerated and uniformly dispersed throughout the cellulose acetate network. Changes in the microstructure are attributed to delayed phase separation of cellulose acetate in the presence of ceramic particles.