Résumé : IT is possible to make a classification of all the different types of mechanisms which correspond to a given order of the initial rate of the homogeneous gas phase thermal decomposition of simple organic compounds 1 if a free-radical chain mechanism is admitted. These types of mechanisms differ in two ways: (a) in the type of radicals which takes part in the chain-breaking step, which can be a bimolecular reaction or a triple collision; and (b) in the chain-initiating step, which can be monomolecular or bimolecular. © 1950 Nature Publishing Group.