An in-depth investigation into the validity of the two-phase model in describing the bulk and suspension polymerization of vinyl chloride monomer is carried out, in the light of the latest theoretical and experimental works that have appeared in the literature. Particular attention is paid to the influence upon the reaction of the transfer of free radicals and of the distribution of the chemical initiators between the two phases. Equations giving, respectively, the kinetic chain length and the degree of polymerization of the polymer, as a function of conversion, are calculated. A good agreement between these theoretical models and the experimental data is observed. Methods for the detemination of the kinetic parameters, and numerical values of some of them are also given.