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The composition of surface films of acetic, propionic, and butyric acids on aqueous and cyclohexane solutions and of acetic and propionic acids on ethyl alcohol solutions have been investigated. Surface tensions and vapour pressures are determined, and the Gibbs adsorption equation used to calculate surface excesses, rZ(Nt. The individual amounts of the components at the interface, are assessed by well-known methods. The results are consistent with a monolayer model of the adsorbed phase with perpendicular orientation of acid for the water and alcohol systems, but with parallel orientation for the cyclohexane systems. The occurrence of a mixed monolayer is observed for all the systems, and a unimonolayer of acid is formed only from pure acid. For the aqueous systems preferential adsorption of acid increases with increasing acid chain length, but for the cyclohexane systems, preferential adsorption decreases with increasing acid chain length and is ' negative ' throughout for the propionic and n-butyric acid systems. The ethyl alcohol systems show preferential adsorption of the alcohol. These are interpreted in terms of the affinities between acid and solvent as deduced from their polar or non-polar nature and from their vapour pressure curves.