Following a brief discussion of various methods of measuring combustion efficiency in gas turbine chambers, the superiority in accuracy of applied gas analysis is stated. Factors leading to inefficiencies in combustion are outlined and the exhaust constituents requiring measurement are deduced. The chief difficulty in the past in the application of gas analysis methods to combustion efficiency determination has been the time consumed in actual analysis of samples; at Thornton Research Centre means of overcoming this difficulty have been investigated and several rapid and accurate analytical techniques have been developed and used successfully. These techniques are described, together with other promising methods at present under development. The broad requirements of obtaining exhaust gas samples for analysis are discussed and typical sampling systems described; precautions necessary in applying these systems are given. Formulse for the calculation of combustion efficiency from analytical results, together with any assumptions made, are given in the text and their derivation explained in an Appendix. The techniques described have been developed primarily for use in conjunction with rig testing of aircraft gas turbine combustion chambers; the broader application—for instance, to engine testing—of the principles involved are considered briefly.