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 M x M′1- x O 1,5 [M′=yttrium (III), indium (III), lanthanum (III), gadolinium (III) or thallium (III)] and M x M′1- x O 2 -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 M 2 O 3 and pyrochlore and relevant crystallographic parameters are measured. It was also attempted to introduce Er (III) in the perovskites SrTiO 3 and BaTiO 3 . LaErO 3 and LaYbO 3 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.