We comprehensively explore the use of mass spectrometry (MS) for quantitative determination of the mode of termination, i.e., the extent of disproportionation versus combination, in radical polymerization. Development of the underlying kinetic theory forms a major portion of our endeavors. What emerges from this theory is that the ratio of polymer from disproportionation to combination is a function of chain length and of fundamental kinetic parameters such as the rate of initiation and the rate coefficients for propagation and termination. Therefore, all this information must be at hand in order to determine λ, the fraction of termination by disproportionation. We implement our findings using electrospray-ionization MS results for poly(methyl methacrylate) that we synthesized at 85 °C in benzene employing bis(3,5,5-trimethylhexanoyl) peroxide as initiator, chosen because it has the advantage of yielding only one primary radical species. Our results are well fitted by theory and are found to vary with chain length and initiator concentration as predicted. We also address the issue that signal intensities from macromolecular MS techniques may not give the true molecular weight distribution of a sample because of variation of ionization efficiency with species mass. We devise a way of obtaining λ from data for hypothetical combination and disproportionation species of identical mass. Our final result is λ = 0.63 ± 0.10, suggesting that disproportionation is not as rampant in methyl methacrylate systems as commonly thought.