Asymmetric membranes with a thin dense top layer of PEEKWC, a modified poly(ether ether ketone), were prepared by the dry-wet phase inversion technique. PEEKWC has good gas separation properties, with O-2/N-2 selectivity of about 6 and CO2/CH4 selectivity of 32. The influence of several experimental variables, e.g. the composition of the casting solution, the casting temperature and the evaporation time before immersion, on the thickness of the skin layer and sublayer were discussed. The preparation conditions were optimized in order to reduce the membrane top layer thickness. The membrane performance was tested in terms of gas permeance and permselectivity for N-2, O-2, CH4, He and CO2. In the case of pinhole defects, membranes were coated with dilute silicone solution. Under optimized conditions, membranes with an open cellular morphology and an ultra-thin dense skin of about 50 nm were obtained, presenting very high gas fluxes. Membranes were furthermore screened for possible use in pervaporation of ethanol/water mixtures.