Sn 3d core-level spectra are measured by X-ray photoelectron spectroscopy (XPS) for nearly monodisperse acidic and alkaline SnO2-x, nanoparticles in the size range 3-4 nm (original) and 1.5-2.5 nm (ultrafiltrated) in diameter. The comparison between the spectra obtained from the acidic and alkaline filtrated samples reveals that the difference is small after a few days but very large after 3 months. The acidic samples show little change in the peak positions and the Sn valence remains around 2.9. The alkaline samples exhibit an increase in the binding energy of more than 0.1 eV and the Sn valence increases up to 3.60 after 3 months. The fact that the lattice strain and the valence change of the Sn ions in the alkaline samples are larger than those in the acidic samples is well explained by surface coating and structure, in which the tin oxalate in the acidic samples is much more stable than the ammonium tin oxide in the alkaline samples. (C) 2002 Elsevier Science B.V. All rights reserved.