Effect of γ-ray irradiation in vacuo and in water at ambient temperature were studied for samples of copoly(ethylene-vinyl alcohol) (EVAL) having various compositions.In vacuo, samples having less than 62mol% ethylene contents were degraded and those having more ethylene contents were cross-linked, when they were irradiated at 100Mrad. In water, however, all the samples with various compositions were cross-linked even at 1 Mrad, and gel fractions were obtained as 69-90wt% at 77 Mrad. The gel fraction depended upon the ethylene content of EVAL samples showing a minimum at ca. 50mol% at any dose.Dose necessary for the gel formation of EVAL should be about 10-50 times of those for PE and PVA homopolymers. These results were explained reasonably in terms of the relation between water swelling behavior and composition of EVAL, and of the gelling conditions of PE and PVA by irradiation.