Crystal structures of ethylene-vinyl alcohol copolymers (EVOH) with various vinyl alcohol (VA) compositions have been determined on the basis of the X-ray diffraction data measured for the uniaxially and doubly oriented sample specimens. The unit cells of EVOH with a VA content higher than 27 mol % are monoclinic of the space group P112(1)/m, in which the two planar-zigzag chains are statistically packed in a mode similar to that of poly(vinyl alcohol) (PVOH). With a decreasing VA content from 27 to 14 mol %, the crystal system approaches the hexagonol type. The unit cells of EVOH with a VA content below 14 mol % are orthorhombic of the space group Pnam, similar to the packing mode of orthorhombic polyethylene (PE). The crystal structure of EVOH with the VA content in the range of 14-27 mol % is the pseudohexagonal type. In this way the transformation of the unit cell structure between the PVOH type (>27 mol %) and PE type (<14 mol %) was found to occur via the appearance of a hexagonal-type structure. This transformation was supported also by the change in the infrared spectral pattern measured for a series of EVOH copolymers.