A study has been made of the adsorption of several esters of dicarboxylic acids both at the alkane/water and the air/water interface. The adsorption of n-butanol and n-heptanol at the air/water surface has also been investigated. The surface pressure (π) – surface area (A) isotherms are compared for the various films, and standard free energies of adsorption have been determined.Attempts have been made to fit the π, A isotherms using surface equations of state based on the models of both a two-dimensional gas and a two-dimensional solution. The solution model has proved reasonably successful for fairly dilute films at the air/water surface. At higher coverages, an equation derived by Smith for liquid expanded monolayers gives a moderately good description of films of heptanol on water. A simple application of the solution model to adsorbed monolayers at the liquid/liquid interface met with little success. It is found however that two-dimensional gas equations describe such systems surprisingly well for fairly low surface concentrations.As part of the study, the activity coefficients of methyl dodecanoate, and the diethyl esters of pentan-1,5-dicarboxylic acid and decan-1,10-dicarboxylic acid, all in dilute solution in n-octane at 30 °C, have been determined.