The binding of DTAC (docecytrimethylammonium chloride) to amphiphilic polyelectrolytes of differing hydrophobicity, the alternated copolymers poly(maleic acid-fo-alkyl vinyl ether) with alkyl = hexyl, octyl, decyl, dodecyl, and hexadecyl, referred to as PS6, PS8, PS10, PS12, and PS16, respectively, has been investigated by potentiometry with a surfactant ion-specifc electrode, by dialysis, by time-resolved fluorescence quenching, by fluorescence anisotropy, and by viscosimetry. In aqueous solution, in the absence of surfactant these polymers can form hydrophobic microdomains and sire referred to as polysoaps. The surfactant binding isotherms, the number of surfactants (N) per hydrophobic aggregate, the lifetime of pyrene (tau(2)) in these aggregates, and their microviscosity (eta(i)) have been obtained as a function of the surfactant concentration, C, and the copolymer neutralization degree alpha. The binding is not cooperative, but the binding constants are very large so that most of the surfactant was bound to the copolymer, under the experimental conditions used, and N is proportional to C. These results differ strongly from those characterizing the interaction between DTAC and the hydrophilic poly(maleic acid-co-methyl vinyl ether) at all alpha’s and poly(maleic acid-co-butyl vinyl ether), or PS4, at alpha greater than or equal to 0.50, where binding is cooperative and N is independent of C. The results indicate that the added surfactant can be considered to be partitioned between the aqueous phase and the microdomains formed by the amphiphilic copolymers, simply swelling these microdomains. They permitted us to obtain the number of polymer repeat units per microdomain In the absence of surfactant. The large values of the pyr ene fluorescence lifetimes have been related to the high microviscosity of the mixed surfactant-polysoap microdomains, which reduces the efficiency of quenching. of pyrene fluorescence by molecular oxygen solubilized in the microdomains. The binding of DTAC to PS4 at alpha = 0.25 has some of the characteristics encountered with hydrophilic polyelectrolytes (N changes only little with C) but is not cooperative and shows no critical aggregation concentration, as for polysoaps. The results are discussed in terms of the structure adopted by polysoaps in solution, prior to the addition of surfactant, and of the models of binding for the two limiting cases presented.