An apparatus (1) for determining the proportion of gases in a gas mixture, comprising: a measurement chamber (2) having a chamber defining structure (5), a gas inlet (8) and a gas outlet (10); an ultrasound source (6A) and an ultrasound detector (6B) mounted such that the ultrasound source is capable of transmitting ultrasound through the chamber to the ultrasound detector; a temperature sensor (14) mounted such that the sensor is capable of sensing the temperature in the chamber; wherein the chamber defining structure is adapted to amplify thermal exchange with a gas content in the chamber so as to suppress a temperature change in the chamber.