This paper examines the relationship between the composition of two-component self-assembled monolayers (SAMs) of alkanethiolates on gold and the composition of the solutions from which they were formed. The SAMs were prepared by competitive adsorption of a long-chain alkanethiol (HS(CH2))(21)CH3) and a short-chain alkanethiol (HS(CH2)(11)OH) from solutions in ethanol. Under conditions in which the alkanethiolates in a SAM and the alkanethiols in solution are close to equilibrium, the relationship between the composition of the solution and the composition of the SAM suggests that the monolayer tends, thermodynamically, to exist as a single phase predominantly composed of either long-chain or short-chain thiolates. A derivation of the thermodynamic relationship between the compositions of the SAM and solution is described that includes intermolecular interactions between components in the SAM; theory and experiment agree qualitatively. This analysis concludes that, for a two-component system of alkanethiolates on gold well-equilibrated with alkanethiols in solution, a single phase is preferred at equilibrium; phase-separated, two-component monolayers of the sort extensively studied in Langmuir systems are not observed.