The method of determination of the thermal hazard properties of reactive chemicals from DSC experiments is illustrated by the results of SADT simulations performed with azobisisobutyronitrile (AIBN). The kinetics of decomposition of AIBN in the solid state was investigated in a narrow temperature window of 72–94 °C, just below the sample melting. The kinetic parameters of the decomposition were evaluated by differential isoconversional method. The very good fit of the experimental results by the simulation curves, based on the determined kinetic parameters, indicated the correctness of the kinetic description of the process. Application of the kinetic parameters, together with the heat balance performed by numerical analysis, allowed scale-up of thermal behaviour from mg- to kg-scale and simulation of SADT. The study presents the evaluation of the influence of the overall heat transfer coefficient U on the SADT value. The results obtained clearly illustrate also the dependence of SADT on the sample mass. The tenfold increase of the mass from 5 to 50 kg results in the decrease of the SADT from 50 to 43 °C. Determination of the reaction kinetics, describing the rate of heat generation, and the heat balance in the system, based on Frank-Kamenetskii approach, was calculated using AKTS Thermokinetics and Thermal Safety software.