The influence of low molecular weight surfactants on the rheology of aqueous solutions of model hydrophobic ethoxylated urethane (HEUR) associative thickeners (AT) under dynamic shear has been studied. Solutions of HEUR-AT behave as Maxwell fluids with a well-defined single relaxation time. It is shown that surfactants profoundly influence the rheological properties displayed. The type of effect observed is dependent on the polymer concentration. For example, in the case of the plateau modulus, at low polymer concentrations, the surfactant produces an initial rise in modulus, which then peaks and gradually falls as the surfactant concentration is increased. At higher polymer concentrations the initial rise and peak are not observed. In the case of the viscoelastic relaxation time, ostensibly similar trends are observed at low polymer concentrations as are observed for modulus. Increasing the polymer concentration, however, does not result in the disappearance of the peak. The relaxation time is also observed to broaden at higher surfactant concentrations. The results described are compared with the predictions of an elementary statistical mechanical model, supported by Monte Carlo simulation, describing the distribution of chain states in relation to the concentration of micelles. The micellar concentration is shown to determine the ratio of chains in looped conformations to those present as bridges. When the effect of surfactant on the total number of micelles is included in the model, the predictions of the theory are found to account for the observed experimental results.