Anabsoluteanalytical procedure is found for obtaining the parameters of the differential, binary, copolymer composition equation, setting up a least-squares condition that places equal weight on all experimental lines of the Mayo-Lewis plot. The values of monomer reactivity ratios for the system ethyl methacrylate (M1-vinylidene chloride (M2), studied by Agron et al., are r1= 2.052 ± 0.043 and r2= 0.346 ± 0.052. These values, especially r1, differ from the estimates by Agron et al. The new solution, however, appears to yield the estimate of maximum likelihood for the reactivity ratios based on the given experimental data.