Experimental measurements have been made of cylinder pressure, crank angle, ignition delay, exhaust-gas temperature and fuel consumption for a 3.9-litre diesel engine operating on air and on mixtures of oxygen, carbon dioxide, argon and helium. The performance of the engine was evaluated over the available load range at speeds of 1600, 1800 and 2000 min(-1), and also under maximum load conditions at various speeds. Results show that the ratio of heat capacities of the gas at the point of fuel injection is the major factor in determining the performance in comparison with natural air operation. A mixture of oxygen, carbon dioxide, argon and helium was prepared which gave an engine performance close to that for operation on air. The measured engine output characteristics did not appear to be influenced by any factor that could unambiguously be attributed to chemical effects of the components of the mixtures.