The concentration of xenon, in a medical gas mixture of xenon as an active component with a known composition of oxygen and, optionally, nitrogen and/or helium recirculating through a medical device (103) in which carbon dioxide is introduced into the mixture, is monitored by removal (135, 133) of carbon dioxide downstream of the medical device (103) and subsequently passing the carbon dioxide-free gas through an ultrasonic gas analyser (143) upstream of the medical device. The gas analyzer determines the xenon concentration by measuring the time delay between transmission of an ultrahigh frequency ultrasonic pulse of at least 100 kHz axially through the sample chamber from a location at one end thereof and reflection of said pulse axially from the other end of the sample chamber back to said location. An analyser of novel construction is disclosed.