Structural and hyperfine properties of Er-doped SnO2 nanoparticles synthesized by a polymer precursor method are reported in this study. X-ray diffraction patterns of samples doped with erbium up to 10 mol % indicate the formation of the tetragonal rutile phase. The mean crystallite size shows a rapid decrease from ~12 nm for the undoped sample down to ~4 nm for the 10 mol % Er-doped sample. Structural changes as a function of the Er content, i.e., changes in the lattice constants, Sn-O bond distances and bond angles, strongly suggest the substitutional solution of Er3+ ions and the onset of oxygen vacancies throughout the SnO2 lattice. No dipolar-magnetic interaction is determined from the Mössbauer spectra which are well resolved by fitting with a distribution of electric-quadrupole doublets. Changes observed in the quadrupole splitting as a function of the Er content have been associated to the local strain induced by the cationic size mismatch and oxygen vacancies. This local strain affects the lattice contribution of the electric field gradient. The linear increase of the isomer shift is assigned to the enhancement of the oxygen vacancies as the Er content is increased. It is found that the oxygen vacancies provide with conduction electrons to the Er-doped SnO2 nanoparticulated system, therefore modifying its electronic properties. © 2012 Springer Science+Business Media Dordrecht.