The interactions between the anionic surfactant sodium dodecyl sulfate (SDS) and a hydrophobically modified nonionic polymer, methyl cellulose (MC), have been investigated in aqueous solution by fluorescence correlation spectroscopy (FCS) and rheology. FCS is used to follow the dynamics of different populations of single aggregates. We are able to follow the solution properties over a wide concentration range of both polymer and surfactant. At constant MC concentration the diffusion time of single aggregates increases gradually up to a certain SDS concentration and decreases to a minimum when the SDS concentration is further increased. This behavior coincides with the behavior of the zero shear viscosity. A model is proposed to explain the effect of surfactant concentration on polymer conformation and aggregation size.