Instantaneous transient gas temperatures were measured in a simulated rapid thermal chemical vapor deposition (RTCVD) reactor. The gas flow consists of an axisymmetric jet impinging on a heated wafer. The jet momentum is opposed by buoyancy from the heated surface. Temperatures were found non-invasively by laser induced Rayleigh light scattering (RLS). A RLS diagnostic system measures temperature by sensing intensity of light scattered from a control volume. Intensity is related to gas molecular number density, and thus temperature can be found from the ideal gas law. The RLS system was calibrated first with measurements in the potential core of a heated jet, and then in the RTCVD test section operating at ambient temperature but reduced pressure. In both cases, the measured intensity of Rayleigh scattered light had low uncertainty. Measured molecular number density corresponded well with that theoretically predicted from the ideal gas law. Transient temperature measurements were then recorded at several radial locations above the wafer surface as it was heated from ambient temperature to 200°C. Operating conditions consisted of Reynolds number Re, = 60, maximum Grashof number GrH = 4.4 × 106, and maximum mixed convection parameter GrH/Rei2 = 1200. Uncertainty of mean transient temperature was ±2°, while temperature fluctuations were large (T′ ∼ 50°C). Both flow visualization and temperature measurements showed that the flow field consisted of buoyancy dominated recirculations, and was highly three dimensional.