THE alternative definitions of combustion efficiency based on the ratio of actual to theoretical heat releases and of theoretical to actual fuel requirements are discussed in relation to the practical need for reckoning enthalpies above a datum temperature. Empirical expressions are obtained which enable the enthalpies of the reactants and the products of combustion to be calculated to an adequate standard of accuracy. The appropriate heat of combustion of the fuel is defined and the effect of temperature examined. The difference in calculated efficiencies obtainable from the alternative definitions are then determined under extreme conditions, and a practical formula developed. Finally, experimental errors in combustion efficiency determination are examined for both the heat balance and gas analysis techniques.