Partially carbonized polyimide (CPI) membranes were prepared from a solution of 30 wt % polyamic acid in NN-dimethylacetamide. The polymer membranes formed on an alumina support were thermally treated, involving imidization in air at 180degreesC and carbonization in N-2 at relatively low temperature (400-500degreesC). The cross-sectional views of the supported CPI membranes show that the membranes consist of a top layer (thickness, 10 mum) on the support and a CPI/alumina thin layer in the support. The CPI membranes carbonized at 500degreesC showed high permeability for O-2 of 1000-30000 barrer and permselectivity for O-2/N-2 of 3-6. The permeability of the CPI membranes was much higher than that of the reported polymer membranes and the carbon membranes. The pores formed under carbonization at 500degreesC and 400degreesC were effective for separating O-2/N-2 and CO2/CH4 mixtures, respectively. TG analysis indicated that the carbonization proceeds even at a constant temperature of 500degreesC. The successive generation of flexible pores before the formation of graphite structure with rigid pores seems to contribute to the higher permeability of the CPI membranes.