An apparatus for the measurement of exhaled respiratory gas temperature during free voluntary tidal breathing, comprises a housing 1 defining a chamber, an air inlet 2 for receiving a stream of exhaled respiratory gas and an air outlet 4 for permitting escape of exhaled respiratory gas from the chamber to outside of the housing. A tube having inner and outer surfaces, is located within the housing and extends from the air inlet into the chamber provides a passageway through which the stream of exhaled respiratory gas may travel from the air inlet in to the chamber. A temperature sensor 6 is located within the housing for measuring the temperature of exhaled respiratory gas. The air inlet, temperature sensor and air outlet are configured such that in use the stream of exhaled respiratory gas traveling within the housing contacts with at least part of the inner surface of the tube when the respiratory gas is traveling upstream of the temperature sensor and contacts with at least part the outer surface of the tube when the stream of respiratory gas is traveling downstream of the temperature sensor.