Most practical synthetic polymers are copolymers. The copolymerization of monomers with different reactivities proceeds through a complicated reaction process, and it is therefore not uncommon that the chemical composition of the resulting copolymer varies between a high molecular weight region and a low molecular weight region. SEC-NMR and DOSY are useful methods for investigating the molecular weight dependence of the composition of a copolymer. In this chapter, SEC-NMR and DOSY experiments on ethylene–propylene–diene rubbers and acrylate copolymers are described and the characteristic features of each measurement method are explained. Applications of SEC-NMR and DOSY using new hardware such as cryogenically cooled probe technology or high field-gradient systems to the analysis of copolymers are also described.