Sum-frequency spectroscopy has been used to study coadsorption of dodecanol and sodium dodecyl sulfate (SDS) at the interface between aqueous solutions and a monolayer of octadecanethiol on gold. Selective deuteration was employed to distinguish between the two adsorbed surfactants. Mixed monolayers were studied over a range of SDS concentrations with dodecanol present at a level of 1 mol %. The results are compared with other data published previously and with literature studies of the air/water interface. A quantitative analysis of the monolayer adsorbed from 6 mM SDS + 10 mu M dodecanol showed that the monolayer comprised 63% dodecanol and 37% SDS and had a packing density comparable to that of a monolayer of pure dodecanol. The composition of the monolayer was found to be isotope dependent. This dependence might arise from a highly surface active impurity in the SDS but can also be explained quantitatively by a lattice model of the monolayer within regular solution theory.