An instrument was built for the permeation testing of flat polymer membranes under pressures up to 3.0 MPa and temperatures up to 300 °C. The high pressure, high temperature cell uses aluminum tape and a graphite gasket to minimize the leak from the high pressure side to the low pressure side, making possible the permeability measurements of slow diffusing gases such as N2. A computer program developed on a LabVIEW platform fully controls the instrument and data acquisition. It incorporates algorithms to automatically adjust the downstream volume, repressurize the upstream volume, vent the downstream volume to prevent over pressurization, and change the temperature of the permeation cell. The percent relative standard deviation of the permeability measurements was <5.5%. Flat membranes of VTEC PI-1388 polymer were tested from 0.3 to 3.0 MPa and from 35 to 300 °C. The permeabilities and fluxes of H2, CO2, and N2 increased with increasing temperature, while the H2∕CO2 ideal selectivity remained unchanged. The major contribution to increased flux arose from increments in temperature rather than pressure.