Using the methods of mathematical simulation, the opportunities for application of a dual salt bath and a dual catalyst bed to the conventional and low air ratio processes of oxidation of o-xylene into phthalic anhydride are studied. It is established that both systems lead to a significant increase of the yield of the raw phthalic anhydride, as well as to a drastic reduction of the undesired side product phthalide. The optimal characteristics of a dual salt bath and a dual catalyst bed are determined. Experimental temperature profiles obtained in an industrial reactor for oxidation of o-xylene during all the time of catalyst use are studied. Both the nature of variation of the catalyst activity along the bed and its influence on the effectiveness of the process are defined.