The results of studying the effect of gaseous products on the kinetics of thermal decomposition of chloride-containing complex ammonium nitrate-based fertilizers using the methods of thermogravimetry and differential scanning calorimetry are presented. The experimental data were analyzed with the Sestak–Berggren model fitting method and the isoconversion differential Friedman method. Based on the results of the studies, the dependences of the decomposition rate on the degree of decomposition are determined and a conclusion is drawn on the reasons of the influence of the self-generated atmosphere on the kinetics of exothermic decomposition.