Mixtures of sorbitan fatty acid eaters and the analogous polyoxyethylene adducts are often used to stabilize emulsions. However, the phase behavior of these mixtures is not clearly understood. In addition, the properties of films of these surfactants depend on solvent conditions. Previous results for mixtures of polyoxyethylene sorbitan monoleate with sorbitan monooleate sorbitan sesquioleate, or sorbitan trioleate showed that these surfactants form nonideal mixtures at the air-water interface. In the work reported here, similar films have been investigated with high ionic strength subphases. Nonideality observed with monooleate- and sesquioleate-containing films is enhanced at elevated ionic strength. For mixed films containing sorbitan trioleate and polyoxyethylene sorbitan monoleate, condensation is observed for films of nearly equimolar composition. A model to explain this behavior is based primarily on molecular shape complementarity.