The validity of the copolymer composition equation for the copolymerization of macromers with small monomers for the preparation of graft copolymers has been examined. The reactivity ratios of high molecular weight macromers r1 cannot be determined experimentally with sufficient accuracy whereas those of small monomers r2 may be calculated by r2=ln(1-p2)/ ln(1-p1), where p1 and p2 are the respective conversions of the macromer and small monomer. A single experimental datum obtained even at high conversions may be used. The error of the method can be readily calculated.