Optical microscopy was used to investigate the phenomena involved in the precipitation of cellulose acetate membranes cast from various binary solutions containing 20 %（wt.） of acetone, acetic acid and dioxane, respectively. It was observed that the high rate of precipitation induced convection flows in the cast polymeric solution during precipitation by cold water. The formation of membrane defects such as voids and large cavities as observed by previous workers could well be attributed to the occurrence of such convection flows. The driving forces for their occurrence could, however, not be properly accounted for by the gradients of buoyant force and surface tension force. The magnitude of the net volume flow rate between the solvent and the non-solvent phases could explain the convection flow. The extension of rate between the solvent and the non-solvent phases could explain the convection flow. The extension of air-exposure period before precipitation was a convenient way for eliminating membrane defects.