Poly(ethylene oxide) (PEO, number-average molecular weight: 2,000,000) was crosslinked by reaction with t-butylperoxybenzoate in the melt. Upon swelling in water, the resulting hydrogels were acidic and suggested clear evidence of spontaneous hydrolysis that continued over periods of several weeks to give clear and low-viscosity aqueous solutions of PEO oligomers. In contrast, in neutral media the gels did not show any signs of hydrolysis. As shown by UV, IR, and size exclusion chromatographic analysis, the PEO hydrolysis products consist of benzoic acid and hydroxyl- and carboxyl end-functionalized low-molecular-weight PEOs. This is consistent with the acid-catalyzed hydrolysis of acetal-, orthoester-, and similar end-functionalized PEOs formed by radical coupling of various PEO radicals with benzoate, alkoxy, and other radicals. Titration of the hydrolysis mixtures indicated that the total molar amount of acid exceeds that of the maximum amount of benzoic acid produced during gel formation. However, the amount of benzoic acid equaled this maximum amount. (C) 2003 Wiley Periodicals, Inc.