In this paper a critical review is made of the methods employed in part I of this series for the determination of the rate coefficients for the copolymerization of styrene and methyl methacrylate. Evidence is presented which shows that the calculations are correct, subject to the normal experimental error. Possible causes of the peculiar variation of the rate of initiation with monomer composition are suggested, based on the idea of recombination of unlike radicals. An analysis of the previous results of Norrish & Brookman, Mayo & Lewis, shows that their results can be interpreted in support of the present work. Some calculations on other copolymer systems seem to indicate that there is a high $\phi $ factor also for these systems, so that the phenomenon is a general one. Finally, a semi-quantitative approach to the problem of calculating, a priori, $\phi $ factors for any system is given; this theory is based on a reduction of the activation energy for the interaction of unlike radicals due to electrical charge effects.