A nasal mask system includes an interface (80) adapted to form an air interface with a patients nose, and a mouth seal (10) adapted to form a seal (20) with the patients mouth. The mouth seal (20) is communicated with the interface (80) via a one-way air path (40) that allows exhausted air from the mouth seal to pass into the interface (80) but prevents air from the interface (80) from passing into the mouth seal (10).
