A new technique which is based on optoacoustic phenomena has been developed for measuring in-cylinder gas temperature and turbulent diffusivity. In the experiments, a high energy Nd:YAG pulsed laser beam was focused to cause local ionization of air at a point in the combustion chamber. This initiates a shock wave and creates a hot spot. The local temperature and turbulent diffusivity are determined by monitoring the shock propagation and the hot spot growth, respectively, with a schlieren photography system.In order to assess the validity and accuracy of the measurements, the technique was also applied to a turbulent jet. The temperature measurements were found to be accurate to within 3%. Results from the turbulent jet measurements also showed that the growth rate of the hot spot diameter can be used to estimate the turbulent diffusivity.In-cylinder gas temperature measurements were made in a motored single cylinder Caterpillar diesel engine, modified for optical access. The measured results were in good agreement with those from multidimensional calculations using the KIVA model. However, the presence of density gradients in the gas due to the compression process was found to deteriorate the schlieren images of the hot spot in the engine, and alternative optical arrangements are being explored for engine turbulent diffusivity measurements.