A number of experiments were made to study tire gas phase reactions that precede TiO2 aerosol formation from TiCl4 in an O2/Ar plasma. The gaseous .species from the plasma-aerosol reactor were detected by a high-resolution QP mass spectrometer. The feed ratio of Ar: O2: TiCl4 was typically 4. l: 0.1. Under such conditions both titanium oxychlorides and oxides of chlorine could be recognized. In the reactor the decay of oxychlorides from the reactions it-as fast, compared to the decay of chlorine. A rough estimate of the quantities of both ClO(g) and Ti0Clx(g) metastable species present is given. TiCl4 oxidation reaction mechanisms with mentioned oxychlorides as intermediates are discussed The theoretical calculations were conducted in tire temperature range from 800 to 3500°C.