A number of experiments were made to study the gas phase reactions that precede TiO2 aerosol formation from TiCl4 in an O-2/Ar plasma. The gaseous species from the plasma-aerosol reactor, were detected by a high-resolution QP mass spectrometer The feed ratio of Ar:O-2:TiCl4 was typically 4:1:0.1. Under such conditions both titanium oxychlorides and oxides of chlorine could be recognized. In the reactor the decay of oxychlorides frp, the reactions was fast, compared to the decay of chlorine. A rough estimate of the quantities of both ClO(g) and TiOClx(g) metastable species present is given. TiCl4 oxidation reaction mechanisms with mentioned oxychlorides as intermediates are discussed. The theoretical calculations were conducted in the temperature range from 800 to 3500 degrees C.