Mouth-breathing can be detected in early phase expiration during respiratory treatments involving a pressure support device. In accordance with some embodiments, an output signal is received from a sensor that conveys information relating to one or more parameters of the pressurized flow of breathable gas as it is delivered to an air-way of a subject. Transitions are estimated between inspiratory phases and expiratory phases of respiration by the subject based on the one or more determined parameters. A local extreme is identified in a level of a first parameter during a given expiratory phase of respiration by the subject. The level of the first parameter is analyzed as a function of time within a sampling window to determine whether the subject is breathing by mouth during the given expiratory phase. The sampling window begins at or proximate to the identified local extreme in the level of the first parameter.
