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An important aspect of the APS plasma spray process is the turbulent mixing of the spray jet with the surrounding air. The air mixing into the jet causes undesirable oxidation of the sprayed coating. In this work the air mixing in the plasma jet was determined by direct measurement of the oxygen content. The measuring method is based on electrochemical determination of the oxygen potential using a solid electrolyte cell. The partial pressure of oxygen along the centreline of the plasma jet was measured with the hydrogen/argon and helium/argon ratio, the gas flow rate and the stand-off distance as experimental parameters. The oxygen content of the plasma tail flame was found to vary between 13.6 to 19.3 % depending on the hydrogen to argon ratio and the stand-off distance. Such high oxygen contents are far too high to avoid serious oxidation of metal coatings. The plasma spraying tests were carried out with WC-Co 17 coating powder. The plasma gases were Ar/H2 and Ar/He. The respective oxygen contents by air mixing were measured to be 18.9 and 17.3 %. The WC-Co was observed to be decarburised more in the Ar/H2 mixture than in the Ar/He mixture, which was attributed to the higher oxygen concentration, higher particle temperature and longer flight time in the plasma jet. Solid electrolyte cell technique was applied to this type of measurements and it proved to be a convenient way to determine the oxygen mixing in the plasma jet.