Light scattering determination of weight-average molecular mass M-w of a copolymer has historically required that measurements be made in at least three separate solvents of different index of refraction. Recently, however, it was demonstrated that, in principle, the evolution of Mw of a copolymer can be followed during copolymerization reactions using automatic continuous online monitoring of polymerization reactions (ACOMP). The continuous measurement of Mw by ACOMP is made possible by the fact that ACOMP provides the comonomer concentrations and absolute light scattering intensity at each instant of time during conversion and, hence, also yields the evolution of the average and instantaneous composition distribution. In this work, a workable and practical formalism for determination of M, during reactions, using a single solvent, is developed for the case of N comonomers. The traditional method quickly becomes intractable since the number of solvents required grows as N(N +1)/2, and the number of auxiliary measurements of differential index of refraction of polymers in the solvents is N(N + 1)(2)/2. The formalism is applied to experimental cases involving copolymerization of styrene and methyl methacrylate, and M. and apparent mass M,P are compared. Results are correlated with traditional multisolvent determinations on endproducts and get permeation chromatography approaches.