The formation of a series of semiinterpenetrating network (SIPN) hydrogels made by free-radical copolymerization of N-isopropylacrylamide (NIPA) and 2acrylamido-2-methylpropanesulphonic acid (AMPS) with varying comonomer mole ratios, crosslinked with N,N'-methylene-bisacrylamide (MBAA) in the presence of polyethylene glycol) (PEG) with average molecular weight 6,000 g mol(-1) was studied via determination of complex viscosity, eta*, using plate-plate rheometry. The isothermal time dependence of,eta* at various temperatures or the variation of eta* with temperature of pregel solutions was utilized to detect the onset c f gelation. The SIPN systems were compared with the corresponding gels made under the same conditions in the absence of PEG. The copolymer mainchain composition has a major effect on the time or temperature for onset of gelation and in particular gelation appears to be inhibited to some extent by MBAA when the AMPS/NIPA mole ratio in the pregel solution exceeds 0.5. The presence or absence of PEG in pregel solutions has a lesser effect on gelation. (C) 2004 Wiley Periodicals.