In the present study carbon molecular sieve (CMS) hollow fiber membranes have been produced by using P84 co-polyimide as precursor material and Argon as inert gas at three different pyrolysis procedures. Specifically, the pyrolysis conditions varied in terms of isothermal time (5,30 and 60 min) at a final pyrolysis temperature of 1173 K. The pure gas permeance measurements of He, H-2, CO2, O-2, N-2 and CH4 purge gases were carried out for all produced carbon hollow fiber membranes (CHFMs) at 333 K. Results show that a longer isothermal time (or the maintenance time) on the higher temperatures results in more selective but less productive CMS membranes. The produced carbon hollow fiber membranes present very high permselectivity values, especially for helium and hydrogen gaseous mixtures. In fact, the maximum ideal selectivity values obtained for He/CH4, H-2/CH4 He/N-2 and H-2/N-2 gas couples were up to 2925, 5500, 350 and 600 respectively. Furthermore, gas mixture experiments present higher selectivity values than the corresponding calculated. (C) 2015 Elsevier B.V. All rights reserved.