Competitive adsorption occurs whenever a solution containing multiple components is in contact with a surface, and the relative adsorption strengths affect both the properties of the interface and the solution. To investigate this phenomenon, the factors influencing relative adsorption strengths of a series of isomeric dialkyl phthalates were determined from their competitive adsorption behavior and interpreted in the context of the thermodynamic properties of the bulk crystals and the structural features of the monolayers. The order of stabilities of the two-dimensional crystals paralleled that of the three-dimensional crystals, as determined by the trends in melting point, enthalpy of melting, and solubility. The magnitude of the stability differences, however, could only be understood through examination of the structures of the monolayers. Thus, the important process of adsorption of solutes from solution must be viewed as a microscopic phenomenon; adsorption strengths are influenced not only by molecular and macroscopic properties but also by the distinct assembly mode of the adsorbed layer.