The structure and composition of a phase-separated arachidic acid (C19H39COOH) (AA) and perfluorotetradecanoic acid (C13F27COOH) (PA) Langmuir-Blodgett monolayer film was characterized by several different types of atomic force microscopic measurements. At the liquid-air interface, surface pressure-area isotherms show that mixtures of the two acids follow the additivity rule expected from ideal mixtures. Topographic images of the deposited monolayer indicate that the surfactants are oriented normal to the substrate surface, and that the acids undergo phase separation to form a series of discontinuous, hexagonal domains separated by a continuous domain. A combination of lateral force (friction) imaging and adhesion force measurements show that the discontinuous domains are enriched in AA, whereas the surrounding continuous domain is a mixture of both AA and PA. This was further verified by selective, in situ dissolution of AA by n-hexadecane, followed by high-resolution topographical imaging of the discontinuous domains.