The extension of the free-radical chain mechanism to heavy paraffinic hydrocarbons has been studied for practical purposes. A generalization of the mechanism is attempted based on the Markov chain theory and presented in a form suitable to computer analysis. This generalization allows easier prediction of the distributions of initial pyrolytic products. Better agreement with experimental results is achieved by taking into account the equilibrium among all the possible isomers for higher homologues of hydrocarbon free radicals. However, the calculation of products is complicated by inclusion of radical isomerization which has not been ascertained experimentally. A new model is constructed herein by avoiding a direct approach to the problem of isomerization, while preserving its effect on the distribution of products. In this treatment, radical isomerization is replaced by a hypothetical process that accompanies the decrease of primary radicals and the increase of secondary radicals. The initial product distributions predicted by the model show good agreement with the observations for a series of normal paraffins from n-butane to n-hexadecane.