Expressions for the molecular size distribution, the average degree of polymerization, etc. are derived by means of the nonsteady-state method for addition polymerization with instantaneous initiation, termination by disproportionation, and combination. Furthermore, a theoretical method is established with which the curves of the differential molecular weight distribution and the values of other molecular parameters can be calculated from the initial conditions of polymerization and the monomer conversion or reaction time. This theory is suited to such types of polymerization as that of vinyl monomers radiated by60Co under deep-freezing conditions, the resultant not very reactive free radical accumulating, then stopping the radiation, and letting the polymerization begin at a higher temperature.