A method to distinguish the mass loss rates corresponding to each of the two decomposition steps in binary blends is proposed. This method is suitable for cone calorimeter test and all other tests measuring continuously mass loss and heat release in an independent way. Heat release rate curves recalculated from the method well fit the experimental ones. Several systems including copolymers, polymers filled with inert or hydrated fillers and binary blends were studied. It appears that the decomposition of these materials is complex because the decomposition rate of one component influences the decomposition rate of the second one. These interactions depend on the materials, their content and also the external heat flux. Concerning binary blends, first results show the decomposition of the most thermally stable polymer is delayed because the temperature within the material is maintained close to the pyrolysis temperature of the least thermally stable polymer. Longer is the delay, higher is the decomposition rate of the most thermally stable polymer. In some cases, this polymer can decompose at higher rate (but later) than the least stable polymer.