The features of oleanolic acid at interfaces and its effect on another component of the epicuticular wax are studied using a binary monolayer system consisting of oleanolic acid (OLA) and stearic acid (SA). A combination of lateral pressure studies, grazing incidence X-ray diffraction (GIXD), and specular X-ray reflectivity (SR) measurements, is proven to be optimal in this respect. In the case of the OLA monolayer, the LS phase exists in the whole accessible pressure region, whereas the SA monolayer develops the phase sequence from the nearest neighbor tilted L2 phase to the next nearest neighbor tilted OV phase and, finally, to the hexagonal LS phase with nontilted molecules with increasing lateral pressure. The cross-section area of 44.1 A2·molecule−1 obtained for the pure OLA monolayer from the position of the diffraction peak agrees well with the circular cross-section of the OLA molecular data. The thicknesses of the OLA monolayer determined by GIXD and XR coincide perfectly. The structure data obta...