Abstract Concepts of gas-turbine plant combustion-chamber efficiency are considered. Three distinct efficiency terms are required, one applying to incomplete combustion and setting losses, one to the flow friction and auxiliary-drive energy requirements, and the third to the influence of these losses on over-all plant thermal efficiency and output. These efficiencies are defined and details of application indicated. The proposed concepts are recommended for consideration as standards since they are thermodynamically meaningful, unambiguous, and relatively easy to apply both in evaluation of test performance and in cycle analysis.