The binding behavior of an anionic surfactant, sodium dodecyl sulfate (SDS), to a series of L-lysine-containing copolypeptides in aqueous solutions was investigated in relation to the conformational change of copolypeptide-surfactant complexes with the use of potentiometric and spectroscopic techniques. The present results of CD spectra and the binding isotherm of SDS by copolypeptides of opposite charges can lead us to conclude that SDS binds cooperatively to the positively charged side groups of a series of copolypeptides used in this work, resulting in the formation of a micelle-like cluster due to an additional hydrophobic interaction among bound SDS ions. Solid-state properties of the stoichiometric copolypeptide-SDS complexes were also examined by using CD and FT-IR spectroscopies; (Lys, Tyr) (1:1)and (4:1) systems adopt a beta-pleated sheet conformation, while (Lys, Trp) (4:1) and (Lys, Phe) (1:1) systems adopt an alpha-helical conformation. Based on the results of FT-IR spectra, in all cases surfactant alkyl chains of SDS in the solid complexes were in an extended conformation.