Aqueous mixtures of sodium dodecyl sulfate (SDS) with QUATRISOFT LM200, a cellulose derivative substituted with cationic hydrophobic side chains, have been investigated by various techniques, in the absence and in the presence of added salt. Steady state fluorescence measurements show that hydrophobic microdomains are formed in aqueous solutions of LM200 already at low concentrations (much less than 1%). On adding SDS to a solution of LM200 in the range 0.02-1%, Liquid-liquid phase separation occurs near charge neutralization (for the same amount of polymer and surfactant charges) for the salt-free mixture and earlier in the presence of salt. In both cases, redissolution occurs upon further SDS addition. The total SDS concentration at redissolution increases linearly with polymer concentration, from a Limiting value, at vanishing polymer content, close to the cmc of the polymer-free solution. Viscosity measurements show that SDS associates to LM200 already at very low SDS concentrations (10(-5) M) and that the binding continues even after redissolution up to the highest investigated ratios of SDS to LM200. In a 1% solution, a very high viscosity is found on both sides of the two-phase area, as previously shown by Goddard and Leung (Colloids Surf. 1992, 65, 211). The results are interpreted in terms of a binding isotherm of surfactant to polymer, analogous to isotherms observed for surfactants binding to proteins or to micelles of other surfactants. The first stages of the isotherm involve binding of individual surfactant molecules to the mixed micelles, and the last stage, occurring when the free surfactant concentration approaches cmc, is a strong and cooperative binding related to the self-association of the surfactant. High SDS/LM200 binding ratios seem required for redissolution and even higher for breaking the micellar cross-links responsible for the enhanced viscosity. Such high binding ratios are only obtained near or within the cooperative binding region, i.e. when the free surfactant concentration is close to the cmc.