Electron spin resonance spectroscopy has been used to investigate the solution properties of a hydrophobically associating cellulosic polymer. Nitroxide spin labels covalently attached to the cellulosic backbone have given information with regard to the segmental motion of the polymer chains, whereas nitroxide spin probes have demonstrated the formation of regions of hydrophobicity above a critical polymer concentration. The data is consistent with the formation of an extensive three--dimensional network in which the hydrophilic cellulosic backbones are effectively cross-linked by the intermolecular association of neighbouring hydrophobic side chains. The work has been extended to study the interaction of the polymer with sodium dodecyl sulphate surfactant and the electron spin resonance data has been used to elucidate the mechanism of interaction, and to explain the unusual rheological behaviour.Hydrophobically associating polymers are finding increasing industrial use due to their ability to impart improved rheological behavior to particulate dispersions (1,2). Consequently, there have been a number of research publications concerning such polymers over recent years (3-13). Essentially, these polymers consist of a hydrophilic backbone and possess a small number of hydrophobic side chains, usually in the range of 8 to 40 carbon atoms in length. Whilst the nature of their backbone usually renders the polymer soluble in aqueous media, intermolecular association of the hydrophobic groups leads to the formation of a weak three-dimensional network structure giving rise to solutions of very high viscosity at low shear rates (8). Addition of surfactants to solutions of hydrophobically associating polymers has also been shown to have a dramatic effect on the rheological properties