The interaction and structure of phases in systems LnF3-Ln'F3 (Ln-Sm, Eu, Yb Ln'-Ce, Tb) are studied at 700-1100°C. Formation of solid solutions on the basis of binary fluorides of lanthanides is established. Appearance of fluorite-like phases of a structure LnF2+xbeginning from 1000°C is observed. The content of the latter notably increases at heat treatment in vacuum. The formation of Ln(II) in systems LnF3-Ln'F3 is confirmed by appearance of intensive bands of absorption in UV range of a spectrum. They are caused by 4f-5d electronic transitions in ions Ln(II) and charge transfer Ln'(IV) Ln(III). The intensity of bands of absorption increases with temperature rise and in a series Sm-Yb-Eu and Tb-Ce, that is in agreement with strengthening of interaction in the studied systems.