The ultra-micropores in polymeric membranes play a key role on the separation performance. This work demonstrates that liberation of small molecules in the membrane formation process is an effective approach to create ultra-micropores in a narrow size distribution in polymeric membranes. A crosslinkable Meldrum's acid derivative is used as the effective agent that liberates CO2 and acetone molecules in the polyimide (PI) membrane formation process and to chemically crosslink the PI-based membranes. Compared to the neat PI membrane, the modified membrane shows small ultra-micropores in a narrow size distribution measured with a positron annihilation lifetime spectroscopy. Consequently, the modified PI membranes become highly-selective toward H-2 gas. With a small reduction of permeability (from 23.1 to 15.5 barrer), the ideal selectivity for H-2/N-2 (155) and H-2/CO2 (5.3) of the modified PI membrane are about 3-fold of the values recorded with the neat PI membrane. In a binary mixed gas test at 20 atm, the modified PI membrane shows a high H-2 permeability of 24.5 barrer and a high H-2/CO2 selectivity 14.1. The results demonstrate an effective and simple method to enhance the gas separation performance of polymeric membranes with modulation of the cavity sizes and size distribution of the membranes. (C) 2015 Elsevier B.V. All rights reserved.