Selective AlPO-18 membranes were reproducibly synthesized on the outside surface of macroporous tubular α-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, N2, CH4 and i-C4H10 decreased with increased kinetic diameter, but CO2 permeated faster than the smaller H2, because CO2 adsorbs more strongly. Eleven AlPO-18 membranes had an average CO2 permeance of (1.8±0.5)×10−7 mol/(m2 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 0.47×10−7 mol/(m2 s Pa) [CO2 flux of 15 kg/(m2 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.