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SYNOPSISIn order to improve the heat stability of poly(viny1 alcohol) (PVA) hydrogel, acetalized PVA was irradiated with electron beam irradiation. The acetalization was strongly affected by PVA water content. There is an optimum water content of 20-30% for acetalization of PVA. The PVA gave maximum tensile strength a t a dose of 100 kGy before or after acetalization. The PVA hydrogel maintained tensile strength of 10 MPa, even after 90 min boiling (98°C) in water or autoclave sterilization (121OC). It was confirmed that irradiation after, or before, acetalization is effective for enhancing heat resistance of a PVA hydrogel.The hydrogel is transparent, soft, has high tensile strength, and high hot water resistance.