The influence of the operating parameters (coolant temperature, inlet temperature of the reaction mixture, initial reagent concentration and gas flow rate) of the process of oxidation of o-xylene into phthalic anhydride taking place in a fixed bed reactor on the hot spot temperature is investigated in this study. Using a two-dimensional heterogeneous mathematical model and physical experiments in a pilot reactor the admissible deviations from the industrial values of the parameters are determined. There is no risk the catalyst to be overheated or a runaway of the hot spot to occur under the variations determined. Two states of the catalyst are investigated-a catalyst which is completely pretreated and calcined, but not pretreated catalyst.