Research on interlayer gallery-based gas and liquid separation has gained widespread attention. A series of layered materials like lamellar ZSM-5, graphene and its derivatives have been fabricated into membranes showing fascinating gas/liquid separation properties. Layered double hydroxides (LDHs) are prominent representatives of layered compounds composed of regularly arranged brucite-like 2D sheets. Here we successfully prepared well-intergrown NiAl-CO3 LDH membranes in one step. Particularly it was found that CO2 dissolved in the precursor solution exerted great influence on the microstructure of prepared membranes. Trace amounts of CO2 in the precursor solution led to the formation of ab-oriented 0.6 mu m thick LDH membranes, while randomly oriented 5 mu m thick LDH membranes formed from CO2-saturated precursor solutions. Both ab-& randomly oriented LDH membranes showed clear size-based selectivity and H-2 was found to preferentially permeate through the interlayer galleries. However, randomly oriented LDH membranes showed a much higher H-2 selectivity possibly due to the decreased density of mesoscopic defects. Furthermore, in addition to the NiAl-CO3 LDH membrane, a compact and randomly oriented ZnAl-NO3 LDH membrane with reasonable gas selectivity was successfully prepared here by proper optimization of the synthesis conditions.