Dense asymmetric gas separation membranes were prepared from a modified poly(ether ether ketone), PEEKWC. Membranes were obtained by the dry phase inversion technique from a polymer/solvent/nonsolvent (PEEKWC/chloroform/1-butanol) solution. The aim of this work was to study the influence of the temperature and the exposition to an air flux on the membrane morphology and transport properties, in order to obtain a thin skin layer, high gas fluxes and high selectivity. The increase of temperature during the casting and evaporation process (from 20 degrees C to 65 degrees C) seems to produce the same effect of the increase of the ventilation above the membranes (at 15 degrees C). In fact, membranes prepared at high temperatures have a thicker skin layer with respect to membranes obtained at low temperatures, and a low porosity in the supporting sub-layer. High ventilation, i.e. a fast air flow above the cast film, causing rapid solvent evaporation during the phase separation process, also produces a thicker skin and a more compact sub-layer. The best membranes in terms of gas flux and selectivities are obtained at 15 degrees C without ventilation during the casting process. For these membranes CO2/CH4 and O-2/N-2 selectivities are 33.8 and 6.7, respectively and the effective skin thickness is about 1 mu m.