A hydrophobically modified alkali swellable emulsion copolymer labeled with pyrene (Py-HASE) was studied by fluorescence, surface tensiometry, and viscometry at concentrations ranging from 0.01 g/L (lower than the overlap concentration, C* = 2.4 g/L) to 10 g/L (above C*) in the presence of the anionic surfactant sodium dodecyl sulfate (SDS). The results obtained by the three techniques lead to the conclusion that binding of SDS onto Py-HASE proceeds in four different stages which are separated by three SDS transition concentrations. The values of the SDS transition concentrations are little affected by the Py-HASE concentration at low Py-HASE concentration but are shifted to higher SDS concentrations at high Py-HASE concentration. The average number of pyrenes per mixed micelle could be determined from the analysis of the fluorescence decays which established a maximum average capacity of pyrenes per mixed micelle of 2.4 +/- 0.5 independent of the polymer concentration. For a Py-HASE concentration of 6.0 g/L, the average number of pyrenes per mixed micelle was found to take the optimal value of 2.0 at the SDS concentration where the solution viscosity peaked. This study constitutes the first example where fluorescence experiments are being used to rationalize the spike in viscosity exhibited by an associative thickener solution upon addition of a surfactant.