Here we report the permeability and separation performance of self-standing carbon molecular sieve (CMS) membranes formed by pyrolysis of polyimide (PI) precursors derived from poly(amic acid) (PAA) with varying intrinsic viscosity. CMS resulted in ultra-microporous membranes showing a classical molecular sieving structure as gas permeation was high for smaller molecular gas (H-2), which then decreased sequentially as the molecular sizes increased in the order of CO2, O-2 and N-2. An important relationship was found when the intrinsic viscosity of the PAA precursor decreased from 1.66 to 0.65 dl g(-1), the ideal gas selectivity jumped to higher values such as from 101.8 to 163.1 for H-2/N-2, from 21.5 to 34.6 for CO2/N-2 and from 6.7 to 10.7 for O-2/N-2 while the permeability decreased such as from 1816 to 1487 Barrer for H-2, from 383.4 to 314.8 Barrer for CO2, from 119.0 to 97.7 Barrer for O-2, from 17.8 to 9.1 Barrer for N-2. The low intrinsic viscosity conferred a superior pore size control of the CMS structure, with an average ultramicropore size around 3 angstrom. The O-2/N-2 and H-2/N-2 ideal selectivity versus permeability results were all above the Robeson's upper bound line, thus demonstrating the effect of low intrinsic viscosity precursors in the synthesis of high performance CMS membranes. (C) 2016 Elsevier B.V. All rights reserved.