The heat effects of the nitration and dissolution processes of 1,2,4-triazol-5-one (TO) in acidic environments were measured by differential reaction calorimetry. The kinetics of nitration of TO in a 200-mL reactor were investigated by UV/Vis spectroscopy. Temperature changes were measured in a 10-L batch reactor during the TO nitration. A model of kinetics for the synthesis of 3-nitro-1,2,4-triazol-5-one (NTO) was proposed and it was used to simulate the phenomena occurring in the calorimeter and in the reactors. The experimental data were compared with modeling results and parameters of the Arrhenius equation for synthesis of NTO with selected nitration mixtures were determined.