Research results are presented for an AC electric arc that burns a mixture of steam and air in a three-phase high-voltage plasma torch and can be implemented to produce plasma for plastic waste gasification. The dependences of electric parameters on the ratio of the steam to air mass flows (H2O/air similar to 1-6) at an approximately constant total mass flow of the plasma-forming gas are obtained during several experiments. During the experiments, the arc parameters were as follows: voltage drop of 1.0-1.8 kV, current of similar to 28.5 A and power of similar to 52-86 kW. The thermal efficiency of the plasma torch was similar to 94-95%. CCD cameras operating at 4000 fps were used to determine the average discharge length of 798 mm. Photography with a high shutter speed (1/8000 s) was used to determine the average arc diameter (similar to 4.47 mm). Arc temperatures were calculated (10,000-11,500 K) using the thermodynamic equilibrium approach. Experimental results indicate that increases in the steam content of the steam-air plasma lead to a reduction of the arc's temperature and electrical conductivity. Using an equilibrium approach, the main parameters of plasma gasification were estimated: syngas yield (3.62-3.48 m(3)/kg), composition (H-2 - 55.5-62.5, CO - 32.8-34.1 vol.%) and energy consumption (11.0-12.3 MJ/kg). (c) 2013 Elsevier Ltd. All rights reserved.