The present paper analyzes the thermolysis gross-kinetics of polypropylene (PP) and a PP-based fire-resistant composition containing 10% of hexabrominecyclododecane (HBCD) as the fire-retardant additive. The kinetic parameters of thermolysis of an individual PP are calculated by the isoconversion method. The increasing dependence of the effective activation energy on the transformation degree has been established. This dependence is attributed to the competitive character of the processes of thermodestruction initiated by oxidation and to the PP pyrolysis. Thermolysis in the helium atmosphere leads to oxidation suppression and increases the effective activation energy at the initial stage of thermolysis. A similar effect can be attained by introducing HBCD into PP, which permits considering the ability of inhibiting the oxidation process proceeding in the condensed phase as one of the reasons of the fire-retardant activity of this additive.