Reflection spectra are measured at 100°K of M= neodymium(III), samarium (III), europium (III), dysprosium (III), holmium (III), erbium (III), and ytterbium (III) in mixed oxides such as MxM′1- xO1,5 [M′=yttrium (III), indium (III), lanthanum (III), gadolinium (III) or thallium (III)] and MxM′1- xO2-0.5 x [M′ = titanium(IV), zirconium(IV) (or mixtures of these two elements), tin (IV) or cerium (IV)]. Most of these oxides crystallize in the disordered fluorite structure or in the two superlattices C-type M2O3 and pyrochlore and relevant crystallographic parameters are measured. It was also attempted to introduce Er (III) in the perovskites SrTiO3 and BaTiO3. LaErO3 and LaYbO3 are distorted cubic (or have a very large unit cell) and certain Nd (III) Ti (IV) and Nd (III) Y (III) oxides present a new, non-cubic, structure. The nephelauxetic effect, the 4 f → 5 d and electron transfer bands in rare earths are theoretically discussed. A comparison is made with Cr(III) mixed oxides.