The viscosity of mixtures of a hydrophobically modified cellulose polymer (HMHEC) and surfactants in aqueous solution was determined as a function of the concentration of mixed micelles. It was shown that, in solutions of low viscosity where the ratio between polymer hydrophobic tails and mixed micelles is low, a high viscosity can be recovered by increasing the surfactant aggregation number in the mixed micelles. The surfactant aggregation number of the mixed micelles was increased by adding either a screening electrolyte or an oppositely charged surfactant to solutions containing the hydrophobically modified polymer and an ionic surfactant. Time-resolved fluorescence quenching (TRFQ) was used to investigate the size of the mixed micelles.