The crosslinking polymerization of acrylamide (AAm) was investigated at a monomer concentration of 5 w/v% in water with N,N'-methylenebis(acrylamide) (BAAm) as the crosslinker. The crosslinker ratio (mole ratio of BAAm to AAm) was set to 1/82. The fractional monomer conversion x, the weight fraction of gel W-g, and the equilibrium weight swelling ratio q(w) of the hydrogels were measured as a function of the reaction time. Experimental results indicate that the first gel molecule in the reaction system appears after a reaction time t(c) = 4 +/- 1 min or after a monomer conversion of x, = 0.07 +/- 0.02. At this critical point the homogeneous reaction system turned into a heterogeneous one consisting of gel and separated solution phases. The network formation process took place in this heterogeneous system within the first 60 min of the reaction time. Experimental x, W-g, and q(w) data indicate that, during the gel growth process, the growing hydrogel molecule cannot occupy the entire polymerization system, which is responsible for the observed phase separation. It was shown that the volume of the separated liquid phase decreases as the polymerization and crosslinking reactions proceed and finally, the reaction system becomes homogeneous again due to the decreasing dilution degree of the hydrogel. (C) 2000 Elsevier Science Ltd. All rights reserved.