The inhomogeneity of the charge in a spark-ignition engine cylinder has been examined by means of a novel fast-response flame ionisation detector.In the experiments presented here propane was used as a fuel, either completely pre-mixed (as a control experiment) or in varying degrees of inhomogeneity, including injection just behind the inlet valve to give a situation similar to that in a gasoline port-injection system. The engine was a single-cylinder Ricardo E6 research engine.The equipment used here provides an insight into the inhomogeneity of the incoming cylinder charge and into the mixing process in the cylinder. The cylinder charge inhomogeneity has been characterised in terms of the average inhomogeneity of the charges, of the cyclic variations therein, and of the cyclic fluctuations in the amount of fuel admitted. (There are, however, limitations to the quantitative information which can be derived from the measurements because of the resolution and delay time of the equipment.)The relationship of these quantities to cyclic variability in combustion was measured. The effect of average charge inhomogeneity on cyclic variability was unclear, but it was found that cyclic fluctuations in the amount of fuel admitted had a prominent effect.