Linear and nonlinear viscoelastic properties of poly(acrylic acid) (PAA) and poly(acrylic acid-co-2-acrylamido-2-methylpropane sulfonic acid)-graft-poly(N,N-dimethylacrylamide) (P(AA-co-AMPSA)-g-PDMAM) mixtures have been investigated as a function of pH, the PDMAM content of the graft copolymer and the molecular weight of PAA. At pH < 3.75, strong hydrogen-bonding interpolymer complexation between PAA and PDMAM side chains in semidilute solution leads to the formation of a transient network, as the considerable increase in viscosity indicates. The sol/gel transition observed at pH = 2.0 by increasing the graft copolymer composition in PDMAM is explained by a substantial increase in the number of the junctions (stickers) resulting from the PDMAAM/PAA hydrogen bonding complexation. Moreover, the thickening effect observed is further strengthened by increasing the molecular weight of PAA, due to the interconnection of more copolymer chains. (C) 2004 The Society of Rheology.