An improved cell which permits the measurement of permeabilities of membranes to gases over a wide range of temperatures and gas pressures is described. The measurements are made by the variable volume method, under constant pressure differential across the membrane. The cell lends itself particularly well to routine tests, because it does not require calibration or the use of vacuum techniques. The performance of the cell is discussed, and typical experimental results are presented. A modified permeability cell of the same type for high-pressure studies is also described. Measurements with this apparatus show that the rate of gas permeation obeys, in some cases, a single from of Fick's law, even under pressure differentials across the membrane as high as 800 psi (54 atm.). The paper also compares permeability data obtained by the variable volume and the variable pressure methods. The permeability of 0.002 in.-thick Alathon 15 polyethylene to oxygen and nitrogen was determined between 0 and 50°C. by the two methods, using the same sample of membrane in situ, and the measurements were found to agree within experimental error. Permeabilites of 0.010 in.-thick samples of Alathon 15 polyethylene to nitrogen, oxygen, helium, and carbon dioxide obtained in the same temperature range by the variable volume method were 15–30% higher than the corresponding data determined by the variable pressure method. This discrepancy could be due to the fact that the variable pressure measurements with the thicker membrances may not have been made under true steady-state conditions, although permeabilities were determined from apparently linear sections of permeated gas pressure vs. time curves. A critical re-examination of the methods used to determine permeability constants is suggested.