Gas temperature profiles in the plume of an argon-hydrogen thermal plasma jet were determined from Rayleigh scattered laser light. Measured profiles were found to be well fitted by a Gaussian curve. Temperature data were compared with values obtained from thermocouples and showed an increasing discrepancy for temperatures higher than 800 K. The presence of a cooled substrate in the flow was observed to increase the center-line temperature by about 22% at the substrate. By combining the temperature results with calorimetric measurements of heat flux, a heat transfer coefficient to a copper substrate held at 300 K was determined to be in the range 400-1000 W/m(2). K under typical plasma spraying conditions.