Poly(N-vinylpyrrolidone) (PVP) hydrogels have become increasingly important materials for pharmaceutical and biomedical applications. UV-light initiated oxidative crosslinking of PVP represents a novel method for producing PVP based hydrogel materials. However, the mechanism of the gelation by this approach is poorly understood. In this study, the reaction mechanism for the crosslinking process is investigated by FTIR, Raman, and solid-state CP/MAS NMR techniques. Both FTIR and Raman spectra indicate that in the process of free radical oxidative crosslinking, the pyrrolidone ring is partially transformed into a succinimide ring. Solid-state NMR data have confirmed this change, and provided evidence that stable intermediates of 4-hydroperoxy-pyrrolidone (PVP-OOH) and its accompanied 4-hydroxy-pyrrolidone (PVP-OH) are formed. The pyrrolidone hydroperoxide intermediate can account for the efficient crosslinking, producing a sufficient level of macroradicals to form stable hydrogels. (C) 2010 Elsevier Ltd. All rights reserved.