AbstractThe following quantities were measured on a number of ethylene–vinyl acetate (EVA) and ethylene–acrylic acid (EAA) copolymers: (1) the small‐angle x‐ray scattering invariant, (2) the overall density, and (3) the crystallinity. Assuming a two‐phase structure, the separate values of the densities of the crystalline and amorphous regions can be calculated from these data. Of these, the crystalline density is compared with the value obtained from the lattice constants. A systematic difference is observed which is ascribed to the presence of comonomeric side groups in the crystalline regions. For the EVA and EAA samples, their concentration is at least 0.3 and 0.5 times the overall concentration, respectively. The amorphous densities are found to be higher than the values calculated from completely amorphous copolymers by extrapolation p