The adoption of Ac2O/HNO3 mixtures for the nitration of organics or the preparation of nitric esters is not free from some risks of explosion. The behaviour of these mixtures at varying Ac2O/HNO3 molar ratios is studied by means of adiabatic and scanning calorimetry. A simplified kinetic model, based on two lumped reactions (one of which leads to the formation of tetranitromethane) is developed to simulate the thermal decomposition of the investigated system under adiabatic conditions. The use of this model allows satisfactory prediction of the temperature profiles in the reactor whereas calculated pressures are generally overestimated due to some model inadequacies to account for the absorption of gaseous decomposition products into the reacting solution.