Asymmetric membrane structures provide greater permeability due to reduced flow resistance and more efficient separation. The aim of this study was to produce a cellulose acetate (CA) microfiltration membrane with an asymmetric structure that combines a prefilter layer and selective layer in one membrane and, secondarily, to investigate how the new CA-based solvent system for manufacturing membranes affects membrane morphology. Asymmetric CA microfiltration membranes were prepared by a single-layer dry-casting process using a pseudo-ternary mixture of CA, methyl formate (MF), 2-propanol and water. The resulting membranes consisted of two isotropic regions with almost uniform pore size covered by a protective skin. Unlike other asymmetric membranes, the selective layer of these developed CA microfiltration membranes faced the solid-liquid interface, while the upper region facing the liquid-gas interface exhibited larger pore sizes. The top layer acted like a prefilter, and the denser bottom layer like a selective layer. Structural studies of the membranes were conducted by scanning electron microscopy. The membrane structures identified are discussed in detail and a theory about the events taking place in the casting solution during membrane formation. (c) 2006 Elsevier B.V. All rights reserved.