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Er 3+ doped SnO 2 xerogels have been obtained from aqueous colloidal suspensions. Emission and excitation spectra were obtained and allowed the identification of two main families of sites for Er 3+ . In the first one Er 3+ substitutes for Sn 4+ in the SnO 2 cassiterite structure. In the second Er 3+ are found adsorbed at the SnO 2 particle surface. For the first family of sites the technological important infrared Er 3+ emission about 1.5 μm is efficiently excited through absorption at the SnO 2 conduction band at 3.8 eV. On the other hand the emission due to adsorbed ions appears inhomogeneously broadened by the statistical distribution of sites available for Er 3+ ions at the surface of the particles. Moreover it is not excited by the host. The emission of this second family of sites could be also excited by an energy transfer mechanism involving Yb 3+ ions also adsorbed a posteriori at particles surface. Results are compared with spectra obtained for Eu 3+ doped samples.