The adsorption of sodium dodecyl sulfate (SDS) and dodecanol from solutions of the pure and mixed surfactants onto hydrophobic substrates was studied by infrared-visible sum-frequency spectroscopy. Pure films of dodecanol were found to be more densely packed and to contain fewer gauche defects than those of SDS. A marked increase in the conformational order of the SDS occurred upon the coadsorption of dodecanol. This effect was only observed below the critical micelle concentration (cmc)-above the cmc, the dodecanol only absorbed weakly and resided mainly in the SDS micelles.