Dynamic light-scattering (DLS) and oscillatory shear experiments have been carried out on aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) and of a hydrophobically modified analogue (HM-EHEC) in the presence of various amounts of sodium dodecyl sulfate (SDS). Both the theological and the DLS measurements revealed significant polymer-surfactant interaction for the EHEC/SDS and HM-EHEC/SDS systems at surfactant concentrations of 8-10 and 4-5 mm, respectively. The observed theological and hydrodynamic effects for both systems were found to be strongly dependent upon the level of surfactant addition, with at first an increase and then a decrease in the values of parameters such as the dynamic viscosity, the storage and loss moduli, the slow relaxation time from DLS, and the wave vector dependence of the slow inverse relaxation time. The strength of interaction was considerably stronger in solutions of the hydrophobically modified polymer at moderate surfactant concentrations due to enhanced hydrophobic associations. At higher surfactant concentrations, a breakdown of chain associations occurred. The theological properties of the hydrophobically modified polymer at moderate surfactant concentrations suggest that the elastic response dominates at higher frequencies. The time correlation data obtained from the DLS experiments revealed, for both systems at all surfactant concentrations, the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. The values of the slow relaxation time were constantly found to be several orders of magnitude higher than those of the fast relaxation time, with a maximum value observed for the HM-EHEC/SDS system at a surfactant concentration of about 4 mm. The fast mode was always diffusive, while the slow mode exhibited a stronger and more complex wave vector dependence. Under certain polymer-surfactant conditions, the DLS results at low scattering angles indicated the formation of large clusters.