A quantitative description of the observed deactivation in the process of oxidation of o-xylene (o-XL) into phthalic anhydride (PA) is given in this paper. Experimental temperature profiles along the length of the fixed bed obtained in an industrial reactor during various periods of the catalyst life are studied. With the help of a two-dimensional heterogeneous mathematical model the degree of the catalyst deactivation along the bed is determined as a function of time. The activation profiles obtained interpret very well the magnitude and the location of the hot spot, the PA yield and the content of the main undesired product phthalide (PH). A terraced change of the catalyst activity along the bed is found out, while a maximum decrease of the activity is registered in the hot spot zone. Optimal temperature regimes valid for different periods of the catalyst life are determined.