The thermal‐induced gelation process in a polyacrylonitrile (PAN) solution containing different amounts of water was investigated through both dynamic and steady‐state rheologic measurements. During the cooling process, the storage modulus G′ was found to intersect with the loss modulus G″ at a temperature which can be considered as the incipient gel point Tig. This crossover point increased with the water content in the PAN solution. On the basis of the power law and the relation , the loss tangent tan δ was traced with decreased temperature at various oscillation frequencies, and a crossover point was observed at the temperature a little higher than the Tig above for all PAN solutions. The water content might affect the value of the relaxation exponent n via two different mechanisms, that is, hydration decoupling of the pendant nitrile groups in PAN at low water content (2 wt% water) and molecule aggregation at high water content (4 and 6 wt%). Nevertheless, the slight difference in the n values reflects the unique structure of the critical gels irrespective of the water level. The apparent viscosity ηa of either the PAN solution or the PAN gel was found to rise with increased water content and both showed pronounced shear‐thinning behavior. Compared with temperature, the water effect on the apparent viscosity ηa is more significant due to the water‐induced aggregation of the molecular chains. Copyright © 2010 John Wiley & Sons, Ltd.