Optical emission spectroscopy has been used to investigate the characteristics of a plasma jet produced by a steam arc cutting torch operated in air at atmospheric pressure. A procedure has been developed for simultaneous determination of temperature and pressure in the plasma jet as well as an effective nonequilibrium factor. It is based on comparison of a few experimental and simulated spectral quantities. The experimental data were obtained from the spectrum of H-beta and OII lines centred at 480 nm. The existence of the shock wave structure characteristic of an underexpanded jet can clearly be deduced from the measured properties. In the first expansion region, the centreline pressure drops from about 1.4 atm at the nozzle exit to about 0.7 atm a few tenths of millimeter downstream. On the contrary, the centreline temperature remains almost unchanged within this region and reaches the value of about 23,000 K.