The binding of SDS to cellulose polymers in the semidilute concentration regime has been studied by means of NMR, ion-selective electrode, and a time-resolved fluorescence technique. Two polymers have been used, differing only in a low degree of hydrophobic modification of one of them. NMR self-diffusion and activity measurements show that the binding of SDS to the nonmodified polymer has a fairly pronounced critical aggregation concentration (cac), while binding to the hydrophobically modified polymer is less cooperative up to a concentration of about the cac in the nonmodified polymer/SDS system. NMR T-2 relaxation and fluorescence studies indicate that surfactants bound to the hydrophobically modified polymers in the noncooperative regime have slow dynamics compared to micellized surfactants, to surfactants bound to the unmodified polymer, and to surfactants bound to the hydrophobically modified polymer in the cooperative regime. Furthermore, in the non-cooperative regime the fluorescence studies imply that the SDS aggregation number of the mixed micelles is low and that the number of hydrophobic zones is invariant with respect to the surfactant concentration.