A detailed study of the gas permeance properties of self-standing graphene oxide (denoted hereafter as GO) membranes, in relation to their pore structure characteristics and their surface chemistry, are reported herein. By varying the filtration rate of the starting GO suspension as well as the amount of the surface functional groups of the employed GO, it was possible to fine tune the pore size of the developed GO membranes. The involved filtration approach led to the arrangement of the two-dimensional GO stacks/layers on the top surface of mixed cellulose ester (MCE) filters with pores of 045 mu m. The as produced films (7-20 mu m thick GO membranes) were easily detached from the MCE substrate and could be incorporated in a membrane module for performing gas permeability studies. Gas transport through the void space between GO stacks, or discontinuities of GO layers, was dominant over transport through the inter layer space. It was demonstrated that the GO porosity strongly depends on the filtration rate: a fast filtration of the GO suspension led to a haphazard arrangement of GO stacks and, as consequence, to a higher porosity; on the other hand, a very slow filtration resulted in more ordered structures, with the individual GO layers arranged one on Lop of the other, the voids corresponding to discontinuities in the stacking of the GO layers along the basal plane. The slow filtration derived membranes were almost impermeable to in-xylene vapor and exhibited a good separation performance for several gas pairs (H-2/N-2, H-2/CO, H-2/CH4, H-2/C2H6, H-2/C4H10 and H-2/SF6) exceeding nearly twice those corresponding to Knudsen type of diffusion. In addition, the more hydrophobic membranes (prepared using GO with a lower concentration of oxygen surface groups) exhibited very high H2O vapor permeances that make them excellent candidates for application in membrane distillation processes. (C) 2015 Elsevier B.V. All rights reserved.