Crosslinked poly(N-vinyl imidazole) (PVIm) hydrogels in the form of rods have been prepared by (CO)-C-60 gamma-radiation initiated simultaneous polymerization and crosslinking of N-vinyl imidazole in bulk and water. In binary aqueous systems, the percentage gelation decreased with increasing water content. The PVIm hydrogels synthesized were further protonated in HCl solutions of different concentration. PVIm and protonated PVIm (H-PVIm) hydrogels have been characterized using spectroscopic and thermal methods, and the swelling behaviour of these two types of hydrogel has been investigated. PVIm hydrogels originally swelled to 600% (by volume) but in their protonated form at pH 7.0 reached 4000% swelling. (C) 2002 Society of Chemical Industry.