The association between the cationic surfactant dodecyltrimethylammonium chloride (DTAC) and a series of hydrophobically modified poly(sodium acrylate) (HMPA) was investigated by theology and by both steady-state and time-resolved fluorescence. The polymers contain 1 or 3 mol % alkyl side groups (dodecyl or octadecyl). The strength of the interaction is improved when the polymer hydrophobicity increases. In the presence of modified polymer, mixed micelles are formed containing surfactants and polymer alkyl groups. Depending on the polymer and surfactant concentrations, the mixed micelles contain alkyl groups belonging to one or more polymer chains. In the later case, cross-linking between polymer chains occurs, leading to the viscosification or gelation of the system. Steady-state and time-resolved fluorescence methods have been used to estimate the aggregation number of the mixed micelles. It was found that the total number of surfactant molecules and alkyl groups in the mixed micelles is close to the aggregation number of the free surfactant micelles. From the above results the number of alkyl groups in the mixed micelles was derived and correlated to the theological behavior of these systems. The importance of inter- and intrachain contributions to the formation of the mixed micelles is discussed.