Molecular dynamics simulations provide microscopic pictures of the behavior of TBP (tri-n-butyl phosphate) at the water−“oil” interface, and in water−“oil” mixtures where “oil” is modeled by chloroform. It is shown that, depending on the TBP concentration and water acidity, TBP behaves as a surfactant, an interface modifier, or a solute in oil. At low concentrations, TBP is surface active and forms an unsaturated monolayer at the “planar” interface between the pure water and oil phases, adopting an “amphiphilic orientation”. Increasing the TBP concentration induces water−oil mixing at the interface which becomes very rough while TBP orientations at the phase boundary are more random and TBP molecules solubilize in oil. The effect of water acidity is investigated with three nitric acid models:  neutral NO3H, ionic NO3- H3O+ and TBPH+ NO3-. The role of these species on the properties of the water−oil boundaries and on the outcome of water−oil demixing experiments is presented. The neutral NO3H form is highly surface active. Hydrogen bonding between TBP and NO3H, TBPH+, or H3O+ disrupts the first TBP layer and leads, at high TBP concentrations, to a mixed third phase or to a microemulsion. These results are important for our understanding of the microscopic solution state of liquid−liquid extraction systems.