The structural modifications of ethylene/vinyl alcohol copolymers under tensile drawing are investigated as a function of draw temperature and draw ratio. By combining wide-angle and small-angle X-ray scattering, it is shown that a mesomorphic-like structure develops from the original crystalline phase for draw temperatures below 110 degrees C and draw ratios beyond h = 4. Thermal analysis indicates that the resulting strain-induced disordered phase reorganises into the stable monoclinic form above 110 degrees C. Annealing treatments reveal that the thermal stability of the new phase is directly connected to the or mechanical relaxation. X-ray and thermal investigations on quenched samples show that the mesomorphic crystalline form can be induced in the isotropic state. A hexagonal unit cell with random distribution of the hydrogen bonds about the chain axis owing to conformational disorder is proposed. A mechanism of crystalline phase modification is discussed in terms of transverse crystallographic slip.