Fabrication, morphology evaluation, and permeance/selectivity properties of three asymmetric BTDA-TDI/MDI copolyimide hollow fiber membranes (HFMs) are reported. The asymmetric HFMs were spun using the dry/wet phase inversion process. The effect of one of the major spinning parameters, the air gap, on the permeance/selectivity properties of the produced HFM was investigated. Scanning electron microscopy was used to evaluate the morphological characteristics and the macroscopic structure of the developed HFM. The permeance values of He, H-2, CH4, CO2, O-2, and N-2 gases were measured by the variable pressure method at different feed pressures and temperatures and the permselectivity coefficients were calculated. The higher selectivity values were evaluated for the M1 membrane and were found to be 49.33, 2.99, 5.13, 5.57, and 9.61 for H-2/CH4, O-2/N-2, CO2/CH4, CO2/N-2, and H-2/CO2 gas mixtures, respectively. The selectivity experiments of H-2/CH4, CO2/CH4, and O-2/N-2 mixtures were performed at 25 degrees C. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4490-4499, 2013