Measurements of mean mole fraction concentration and the rms of the corresponding fluctuations of Freon gas have been measured in an engine configuration where the Freon simulates injected fuel. A Rayleigh-scattering system has been developed to permit the measurements which were obtained with engine conditions defined by a compression ratio of 3.5, a rotational speed of 200 rpm and flat and re-entrant pistons. The results show that the geometry of the fuel nozzle is important 10 the spread of the jet with convergent (spray) nozzles giving rise to improved mixing. The combined effects of swirl, with swirl number of 1.73, and the re-entrant piston is to increase mixing so that values of concentration on the centre line of the fuel jet are reduced by around 50 percent.