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ABSTRACT To optimize the CO2 permeation and CO2/H2 separation performance of hollow fiber-supported polydimethylsiloxane (PDMS) membranes, the effect of the viscosity of the PDMS coating solution on surface morphologies, thickness of PDMS layer, and solution intrusion into surface pores of hollow fiber supports was investigated. Increases in both stirring time and standing time could increase the viscosity of the PDMS solution. The PDMS layer thickness increased when the coating solution viscosity increased, whereas the surface roughness of the PDMS layer markedly decreased and then slightly changed. Moreover, when the stirring time of the PDMS coating solution was 9 min and the standing time was increased from 2 min to 25 min, the CO2 permeance first decreased, then increased to ∼2250 GPU probably due to the decreased intrusion depth, and finally decreased because of the substantially increased thickness of the PDMS layer. However, the CO2/H2 selectivity increased to 3.4 with an increase in coating solution viscosity. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45765. [ABSTRACT FROM AUTHOR]