The denaturation of bovine serum albumin (BSA) by mixed surfactant systems was studied by examining surfactant adsorption on BSA. Polyoxyethylenealkylether (alcohol ethoxylate : AE) inhibited SDS-binding to BSA with consequent denaturation of BSA. The higher the concentration of AE or the larger the adduct number of oxyethylene (EO), the greater was the inhibitory effect of AE on SDS-induced denaturation of BSA. The adsorption of AE on an electrostatic surface significantly increased when SDS was present. The enthalpy of adsorption of the SDS-AE mixed system on BSA as determined with a microcalorimeter clearly differed from that of pure SDS or AE. The AE effect on SDS-induced BSA denaturation is thus due to the adsorption of AE on the alkyl chain of SDS electrostatically bound to BSA. The adsorption of AE apparently stabilizes the alkyl chain of SDS electrostatically bound to BSA, with a consequent decrease in the binding of SDS to BSA.