SummaryXenon is an anaesthetic and possibly neuroprotective gas that is impossible to measure using conventional anaesthetic gas analysers. We compared the performance of two commissioned xenon analysers using ultrasonic and thermal conductivity principles against a reference method of laser refractometry. An experimental gas circuit was constructed and xenon concentrations compared over a range of 0-100% in oxygen. Eighty-two paired measurements were made comparing the experimental methods with laser refractometry. The ultrasonic method displayed good agreement with laser refractometry, with a mean difference of ) 0.74% and two standard deviation limits of agreement of + 1.08% to ) 2.56%. The agreement between laser refractometry and thermal conductivity was poor, the mean difference being ) 5.37%, with two standard deviation limits of agreement of + 0.6% to ) 11.3%. The ultrasonic method for measuring xenon concentrations can be used in breathing circuits. The thermal conductivity instrument may need further development.