Flat-sheet membranes were obtained by immersion precipitation from a polyethersulfone-dimethylacetamide-nonsolvent additive-water system. The effects of propionic acid, propionic acid anhydride, butyric acid and butyric acid anhydride, as an additive in a polymer solution, in relation to the phase separation process and membrane structure and properties were investigated. According to scanning electron microscopy analyses, it was found that a nonsolvent additive has a significant influence on structure of the membrane skin layer and the sublayer.