In the process of extraction of the rare earth metals europium and samarium by D2EHPA, we find that solid films form at the organic/aqueous interface. For extraction solution systems of europium and samarium, we specified concentration zones where the solid films should form, and show that the formation of the solid films occurs at the specific zones of hydrogen ion and extractant concentrations. We also determine the atomic constituents of the solid films by inductively coupled plasma emission spectroscopy, and find that the solid films are crystals composed of neutral 1:3 intermediate complexes. We further investigate film formation in the extraction solution system that contains two lanthanide elements, europium and samarium, We determine the atomic ratio of the solid film, and show that the atomic ratio takes the value which is near to that predicted from the separation coefficient in the organic/aqueous solution system.