Method of determining the composition of polymer mixtures comprising defined polymer components of differing mean molar masses M.sub.n, which comprises the steps (a) recording of at least one .sup.1H-NMR relaxation curve of the polymer mixture and (b) calculating the proportions of the polymer components by matching the measured relaxation curve with the relaxation curves of the individual polymer components. In this way, the composition of polymer mixtures can be determined quickly and simply. Furthermore, process integration for regulating the polymerization is possible.