A system configured for detecting mouth-breathing in early phase expiration during respiratory treatments involving a pressure support device, the system comprising: a pressure generator configured to generate a pressurized flow of breathable gas for delivery via an interface appliance to the airway of a subject; a sensor configured to provide an output signal conveying information relating to one or more parameters of the pressurized flow of breathable gas as it is delivered to the airway of the subject; and one or more processors configured to execute computer program modules, the computer program modules comprising: a parameter module configured to determine one or more parameters of the pressurized flow of breathable gas as it is delivered to the airway of the subject based on the output signal, the one or more parameters comprising a flow of breathable gas passing through the interface appliance; a breath delineation module configured to estimate transitions between inspiratory phases and expiratory phases of respiration by the subject based on the one or more parameters determined by the parameter module; a parameter extrema module configured to identify a local extreme in a level of the flow of breathable gas during a given expiratory phase of respiration by the subject; a mouth-breathing detection module configured to determine whether the subject is breathing by mouth during the given expiratory phase by analyzing the level of the flow of breathable gas as a function of time within a sampling window, the sampling window beginning at or proximate to the identified local extreme in the level of the flow of breathable gas; and a filter module configured to provide a filtered signal by suppressing high-frequency components of the flow of breathable gas as a function of time; characterized in that the breath delineation module is configured to estimate transitions between inspiratory phases and expiratory phases by identifying points where the filtered signal intersects the flow of breathable gas as a function of time when the filtered signal and the flow of breathable gas as a function of time are plotted together. a pressure generator configured to generate a pressurized flow of breathable gas for delivery via an interface appliance to the airway of a subject; a sensor configured to provide an output signal conveying information relating to one or more parameters of the pressurized flow of breathable gas as it is delivered to the airway of the subject; and one or more processors configured to execute computer program modules, the computer program modules comprising: a parameter module configured to determine one or more parameters of the pressurized flow of breathable gas as it is delivered to the airway of the subject based on the output signal, the one or more parameters comprising a flow of breathable gas passing through the interface appliance; a breath delineation module configured to estimate transitions between inspiratory phases and expiratory phases of respiration by the subject based on the one or more parameters determined by the parameter module; a parameter extrema module configured to identify a local extreme in a level of the flow of breathable gas during a given expiratory phase of respiration by the subject; a mouth-breathing detection module configured to determine whether the subject is breathing by mouth during the given expiratory phase by analyzing the level of the flow of breathable gas as a function of time within a sampling window, the sampling window beginning at or proximate to the identified local extreme in the level of the flow of breathable gas; and a filter module configured to provide a filtered signal by suppressing high-frequency components of the flow of breathable gas as a function of time; characterized in that the breath delineation module is configured to estimate transitions between inspiratory phases and expiratory phases by identifying points where the filtered signal intersects the flow of breathable gas as a function of time when the filtered signal and the flow of breathable gas as a function of time are plotted together. a parameter module configured to determine one or more parameters of the pressurized flow of breathable gas as it is delivered to the airway of the subject based on the output signal, the one or more parameters comprising a flow of breathable gas passing through the interface appliance; a breath delineation module configured to estimate transitions between inspiratory phases and expiratory phases of respiration by the subject based on the one or more parameters determined by the parameter module; a parameter extrema module configured to identify a local extreme in a level of the flow of breathable gas during a given expiratory phase of respiration by the subject; a mouth-breathing detection module configured to determine whether the subject is breathing by mouth during the given expiratory phase by analyzing the level of the flow of breathable gas as a function of time within a sampling window, the sampling window beginning at or proximate to the identified local extreme in the level of the flow of breathable gas; and a filter module configured to provide a filtered signal by suppressing high-frequency components of the flow of breathable gas as a function of time; characterized in that the breath delineation module is configured to estimate transitions between inspiratory phases and expiratory phases by identifying points where the filtered signal intersects the flow of breathable gas as a function of time when the filtered signal and the flow of breathable gas as a function of time are plotted together.