Selective AlPO-18 membranes were reproducibly synthesized on the outside surface of macroporous tubular a-alumina supports in a single hydrothermal synthesis step. The AlPO-18 crystals and membranes were characterized by SEM, XRD, adsorption and permeation measurements. Single-gas permeances of CO2, N-2, CH4 and i-C4H10 decreased with increased kinetic diameter, but CO2 permeated faster than the smaller H-2, because CO2 adsorbs more strongly. Eleven AlPO-18 membranes had an average CO2 permeance of (1.8 +/- 0.5) x 10(-7) mol/(m(2) s Pa) and average CO2/CH4 separation selectivities of 101 +/- 16 at room temperature and a pressure drop of 0.1 MPa for the equimolar CO2/CH4 mixture. The membrane was also selective (selectivity =31) at 2 MPa feed pressure and had a CO2 permeance of 0.47 x 10(-7) mol/(m(2) s Pa) [CO2 flux of 15 kg/(m(2) h)]. The CO2/CH4 selectivities decreased as temperature and pressure drop increased. The membranes stability was investigated for different calcination times, calcination temperature ramps, and following exposures to water vapor and liquid. (C) 2014 Elsevier B.V. All rights reserved.