A respiratory gas delivery system (10) is disclosed. The respiratory gas delivery system (10) comprises a wearer interface (16) through which respiratory gas is supplied to a wearers airway and a gas conditioning device (24) for conditioning the respiratory gas upstream of the interface (16) in a direction of gas flow to the wearer, the gas conditioning device (24) comprising a controller (78), wherein the interface (16) comprises a sensor for measuring an operating variable in use such that the controller (78) controls the operation of the conditioning device (24) based on said sensor measurement.
