Protein-surfactant interactions are investigated by following the phase equilibria of three lysozyme-sodium alkyl sulfate-water systems with surfactants of varying alkyl chain length, C10SO4, C8SO4, and C6SO4, within the concentration range 20 wt % lysozyme, 20 wt % surfactant, and 80-100 wt % water. Phase behavior similar to that for the system lysozyme-C12SO4-water is observed.(1) The phase diagram of the C10SO4 system is dominated by a solution phase, a gel phase, and a multiphase precipitation region. For the C8SO4 and C6SO4 systems a solution phase, a precipitation region, and multiphase regions including gel are identified. However, no single gel. phase is observed. Surfactants are sparingly soluble in aqueous solutions of lysozyme. Within the surfactant series the solubility is increased in the following order : C12SO4 < C10SO4 < C8SO4 < C6SO4. The composition of the neutral precipitate is determined to be about 8 surfactant molecules per protein molecule. The extension of the precipitation region toward higher surfactant concentrations is strongly dependent on the surfactant chain length; the shorter the chain length, the larger the precipitation region. This is explained by the maximum yield of precipitate being stable upon addition of a certain amount of excess surfactant, which is concentrated in the supernatant. The redissolution of the precipitate is induced by surfactant aggregation. The concentration of surfactant in the supernatant for the dissolution of the precipitate is less than the surfactant critical micelle concentration in water. The experimentally determined phase diagram can be understood qualitatively in terms of electrostatic and hydrophobic effects.