We have investigated the effect of O-2 and reducing gases (CH4 and CO) exposure on nanocrystalline SnO2 in vacuum and at elevated temperatures (120 degreesC) using three different techniques: X-ray photoelectron spectroscopy, in vacuum resistance measurements and scanning tunnelling microscopy and spectroscopy. XPS and resistance measurements showed that O-2 chemisorbtion causes an upward surface band bending of 0.2 eV and a resistance increase of 50 MOmega while CH4 exposure resulted in a 0.1 eV downward band bending and a 20 MOmega drop in resistance. Stable STM imaging at 120 degreesC was achieved and clearly resolved the 8 nm particles. STS measurements indicate a change in the surface electronic properties of the SnO2 particles following O-2 exposure. (C) 2004 Elsevier B.V. All rights reserved.