The Rayleigh light scattering technique has been used to quantify the mean and fluctuating concentration of a passive scalar used to simulate fuel injection in a reciprocating, two-stroke model engine motored at 200 rpm in the absence of compression. The transient concentration field, which results from injection of Freon-12 vapour through the centre of an axisymmetrically located permanently open valve, has been investigated for injection timings of 40 deg. before and at top-dead-centre as a function of spatial position and crank angle.The purpose-built Rayleigh system, with gated digital data acquisition and software dust particle filtering, was first evaluated in a Freon-12 free jet by comparing results to those obtained with a sampling probe. At low concentration fluctuations and independent of particle density the agreement between the two methods is excellent but at high concentration fluctuations and particle density the Rayleigh system overestimates the Freon-12 mole fraction by up to ∼10% for reasons which are discussed.The results obtained in the model engine indicate that the Freon-12 concentration field expressed in terms of ensemble-averaged mole fractions and rms of concentration fluctuations, is dominated by the high momentum transient jet which, in the near field, exhibits similar trends to the steady jet. Impingement of the jet onto the flat piston improves mixing giving rise to nearly uniform concentration fluctuations of ∼10%.