The nitration of (substituted) benzoic acid esters is usually performed in fed-batch operation. The ester is solved in Sulfuric acid charged first. The nitric acid dosed keeps the reaction temperature under control. The safety regarding uncontrolled reaction conditions was investigated under variation of the ratio of nitric acid to charged aromatic compound. A significant decrease in thermal stability of the reaction mixture was observed with excess of nitric acid. Safe operation cannot be assumed in general when the acid is close to or exceeds 10 % molar ratio. The investigation was restricted to aromatic carboxylic acid alkyl esters which form homogeneous mixtures with sulfuric acid. The thermal stability was investigated by differential thermal analysis (closed systems), adiabatic storage tests, and dynamic calorimeters. The reaction products were analyzed by means of NMR and mass spectrometry. The experiments showed that the formation of dinitro compounds cannot cause the thermal instability alone. The formation of dinitro compounds starts at temperatures above the reaction conditions. Another component is responsible for the thermal instability which accumulates in the presence of an ester group at excess nitric acid. It could be deduced that the decomposition reaction is primarily caused by the reaction of surplus nitryl cation (NO2+ ion) with the nitrated carboxylic acid ester in the presence Of sulfuric acid. The decomposition temperature of the reaction Mixture depends on the nature of the ester group and the type and number of further substituents oil the aromatic ring.