A study has been made of the effect of mechanical activation of a magnesium-molybdenum catalyst modified with zirconium and potassium oxides and an n-dodecane-water system on the conversion of this hydrocarbon in the presence of oxygen. It has been established that ＇＇activation＇＇ of the MgO-MgMoO4 catalyst leads to an increase in the yields of C6-12 olefins (to 25%) with a selectivity of 50-60%, and also to intensification of the cracking and oxidizing ability of the catalyst. ＇＇Activation＇＇ of the n-dodecane-water system is characterized by high selective formation of C6-12 olefins (up to 90%) with an olefin yield of up to 16% and by considerable retardation of cracking of n-dodecane and its conversion products. It is assumed that ＇＇activation＇＇ of the components of the catalyst promotes oxidative dehydrogenation of paraffinic hydrocarbons to olefins, and also their formation by means of simple dehydrogenation and cracking of hydrocarbons present in the reaction medium.