The development of a high-velocity, high-temperature argon plasma jet issuing into air has been investigated. In particular the entrainment of the surrounding air, its effect on the temperature and velocity profiles and the subsequent mixing and dissociation of oxygen has been examined in detail. The total concentration of oxygen and the velocity and temperature profiles in the jet were obtained from an enthalpy probe. High-resolution Thomson scattering provided an independent measure of plasma velocity and temperature, validating enthalpy probe measurements and providing non-intrusive measurements near the nozzle exit. The concentration of atomic oxygen was obtained from 2-photon laser induced fluorescence. Molecular oxygen concentration and temperature was obtained from coherent anti-Stokes Raman spectroscopy. It was found that both the incompleteness of mixing at the molecular scale and the rate of oxygen dissociation and recombination affects jet behavior. (C) 2003 Elsevier Ltd. All rights reserved.