The polymorphism and side group location of a series of ethylene-vinyl acetate copolymers (EVA) and their hydrolytic products (EVOH) have been investigated by high-resolution cryogenic Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. The experimental results indicated that FTIR spectra can provide direct evidence for side groups entering the crystal lattice of ethylene segments. Through the characterization of the specific IR band variation, it was proved that the hydroxyl group of EVOH enters the crystal lattice, while the side groups of EVA exist predominantly in the amorphous region. The polymorphism and the crystalline phase transformation were investigated for the two series of ethylene copolymers. It was proved that there is only an orthorhombic crystalline phase for all the EVOH copolymers. For EVA copolymers, however, besides the occurrence of a normal orthorhombic crystalline phase, a monoclinic crystalline phase was also detected for the samples with a relatively high content of side groups. The monoclinic crystalline phase is a substable structure and is mainly affected by the content of the side groups and the crystallization conditions. The content of monoclinic crystals does not increase linearly with branching, but to some extent, the increase of the side group content is propitious to the formation of a monoclinic crystalline state. Different heat-treatment methods have great influence on the polymorphism of EVA copolymers. With the increase of the annealing temperature, the monoclinic crystal was gradually transformed to an orthorhombic crystal.