Phospholipid monolayers play a critical role in the structure and stabilization of biological interfaces, including all membranes, the alveoli of the lungs, fat droplets in adipose tissue, and lipoproteins. The behavior of phospholipids in bilayers and at an air-water interface is well understood. However, the Study of phospholipids at oil-water interfaces is limited due to technical challenges. In this Study, egg phosphatidylcholine (EPC) was deposited from Small unilamellar vesicles onto a bubble of either air or triolein (TO) formed in a low-salt buffer. The Surface tension (gamma) was measured using a drop tensiometer. We observed that EPC binds irreversibly to both interfaces and at equilibrium exerts similar to 12 and 15 mN/m of pressure (Pi) at an air and TO interface, respectively. After EPC was bound to the interface, the unbound EPC was washed Out of the cuvette, and the Surface was compressed to study the Pi/area relationship. To determine the Surface concentration (Gamma), which cannot be measured directly, compression isotherms from a Langmuir trough and drop tensiometer were compared. The air-water interfaces had identical characteristics using both techniques; thus, Gamma on the bubble can be determined by overlaying the two isotherms. Both TO and EPC are surface-active, so in a mixed TO/EPC rnonolayer, both molecules will be exposed to water. Since TO is less surface-active than EPC, as Pi increases, the TO is progressively ejected. TO understand the Pi/area isotherm of EPC on a TO bubble, a variety of TO-EPC mixtures were spread at the air-water interface. The isotherms show all abrupt break in the Curve caused by the ejection of TO from the monolayer into a new bulk phase. By overlaying the compression isotherm above the ejection point with a TO bubble compression isotherm, Gamma can be estimated. This allows determination of Gamma of EPC on a TO bubble as a function of Pi.