'Instantaneous' hydrogels from poly(N-vinyl-2-pyrrolidone) aqueous solutions can be produced using Fenton reagents, i.e., a redox reaction. A firm hydrogel is formed within 5-10 s, with a total gelling time below 8 min. Hydroxyl radical is recognized as the possible high energy intermediate responsible for the crosslinking reactions. The major variables of the process (PVP, H2O2 and Fe2+ initial concentrations, as well as pH) were examined and shown to operate within a rather narrow working window. The process efficiency and the properties of the hydrogel formed are discussed and compared with other methods of hydrogel production. (c) 2006 Elsevier Ltd. All rights reserved.