The theological behavior and hydrophobic association of hydrophobically modified polyacrylamide with 0.5 mol% of hydrophobic n-nonylacrylamide units were studied in the vicinity of the overlap concentration. It was shown that the theological properties of aqueous polymer solutions are determined by the formation of hydrophobic domains playing a role of physical cross-links between polymer chains. It was observed that a pronounced increase of the viscosity above the overlap concentration is accompanied by the increase of the concentration of hydrophobic domains, but the average size of one domain remains constant. (C) 2003 Elsevier Science Ltd. All rights reserved.