Temperature sensitive N-isopropylacrylamide (NIPA) hydrogels were prepared by free-radical crosslinking polymerization of NIPA in the presence of various N,N'-methylenebisacrylamide (Bis) as a crosslinker in aqueous solution at 22 degreesC. Photon transmission technique at 550 nm wavelength was used to study the spinodal decomposition of these NIPA gels prepared with various molar concentrations of crosslinker (Bis). During the phase transition experiments temperatures of the gel samples increased from 20 to 40 degreesC with 0.4 degreesC min(-1) rate. It was observed that the increase in turbidity is much faster in the gel prepared at lower Bis content than higher Bis content sample which indicated that NIPA-water system reaches the spinodal decomposition much faster for the gel prepared with low Bis content. It is understood that high Bis content NIPA gel posses more heterogeneities which are gained during gelation, present low spinodal temperature, T-s. However, NIPA gels prepared with lower Bis content go to spinodal decomposition at higher T-s values. (C) 2003 Elsevier Science B.V. All rights reserved.