A mouth seal assembly for use with a nasal mask system includes a mouth seal adapted to form a seal with the patient's mouth. The mouth seal is substantially independent from a supply of pressurized air from the nasal mask system. An anti-asphyxia valve may be provided to either the mouth seal over the patient's lips or the nasal mask system. A strap arrangement may support the mouth seal in a desired position on the patient's face in use.
