Abstract: A drying sample line for coupling a patient's expiratory gases to a gas analyzer consists of first and second concentrically disposed tubes where the innermost tube is fabricated from Nafion plastic. The expiratory gas is drawn through the lumen of the Nafion inner tube and, simultaneously, dried air is made to pass through the lumen of the outer tube in a counterflow direction relative to the expiratory gases. Because of the properties of the Nafion plastic, water vapor (moisture) contained in the expiratory gas being coupled to the analyzer passes through the wall of the Nafion tube and into the dried air stream. In this fashion, the water vapor is removed from the expiratory gas mixture being applied to the analyzer.