Abstract: A moisture trap for use in a patient monitoring system, includes hydrophilic material contained within a housing for separating moisture from the patient's exhalations prior to those exhalations reaching the actual monitoring device. The housing is separated into a reservoir containing hydrophilic medium and a passageway through which the exhalations flow. The reservoir is isolated from the passage except for a restricted passage, that restricted passage providing the only exposure of reservoir hydrophilic material to moisture and gaseous medium flowing through the passageway. A hydrophilic body is also positioned in the passageway in the path of exhalation flow. A wick of hydrophilic material in the restricted opening connects the passageway and hydrophilic body in the passageway to the hydrophilic reservoir material. A hydrophobic member is positioned in the flow path of the exhalations to occlude and interrupt the flow path when the hydrophilic material becomes saturated and can no longer pick up moisture.