A systematic study of foam stability in the presence of model nonpolar oils and their mixtures with oleic acid was conducted for two commonly used surfactants (anionic and nonionic) under neutral and alkaline conditions with different amounts of dissolved calcium. In some cases insoluble calcium soap or microemulsions formed in situ. Measurements of the rate of collapse of a foam column were supplemented by microscopic observations of individual foam and "pseudoemulsion" films and by measurement of equilibrium and dynamic surface and interfacial tensions. In the absence of calcium soap use of equilibrium values of conventional entry E, spreading S, and bridging B coefficients was adequate to explain the effect of oils on stability of foams containing the nonionic surfactant. For the anionic surfactant E must be modified to account for electrostatic repulsion in the pseudoemulsion film. Calcium soap particles at the oil-water interface facilitate entry of oil drops and the associated bridging of foam films or Plateau borders, producing a substantial antifoam effect. When this synergistic effect occurs, conventional values of E govern oil entry. In some cases for oils that were mixtures of triolein and oleic acid foam is unstable during foam formation and initial foam drainage but stable at later times, behavior which is explained by calculating transient values of E, S, and B. Addition of n-dodecanol produced significant stabilization of foams of the anionic surfactant containing dispersed oil drops both when calcium soap was present and when high levels of calcium had destabilized the foam at neutral pH. ESB theory proved useful in predicting this effect. An amine oxide surfactant was less effective as a foam booster. In the absence of oil, calcium soap particles can destabilize foams of both surfactants. A model for predicting the precipitation boundary including the enhancement of calcium content in the electrical double layers of surfactant micelles yielded results in agreement with experiment. Foam was less stable while precipitation was occurring (hours) than at equilibrium, perhaps because calcium oleate, which initially formed at the interfaces, was extracted as oleate concentration decreased to its equilibrium value.