The preheating of metallic substrates before powder deposition in air plasma spraying improves the adhesion of oxide coatings, provided heating is performed with an optimal procedure to avoid a too high oxidation state of the surface. It means that the temperature level and heating time have to be monitored carefully. In these conditions a thin layer (<100 nm) of oxides is formed on the substrate surface, the resulting contact of the molten droplets impinging the hot substrate is excellent (Rth<10-7 m2.K/W) and the adhesion properties of coatings are enhanced. This paper is devoted to the study of the metallic surface. The substrate heating and thus oxidation are obtained with a d.c. Ar-H2 plasma jet flowing in air of which the stand off distance is maintained at 100 mm. The parameters investigated are macroscopic surface temperature and heating time. The characterization of the oxide layers is achieved by Mossbauer spectroscopy, near grazing X-ray diffraction, near UV-Visible-near IR spectroscopy and specular reflection infrared spectroscopy. At the end of this paper an attempt will be made to correlate these characterizations to the splats microstructure and coatings adhesion.