The SnO2 material prepared by sol-gel method was modified by thiourea solution in different concentrations (0.05, 0.1 and 0.2 mol dm(-3)). Then the structure and the average grain size of the SnO2 material were investigated by X-ray power diffraction. In order to understand the nature of the species on the SnO2 surfaces, the thermal gravimetric and differential thermal analyzer (TG-DTA) and IR spectra of these modified and unmodified sample were taken. The result indicates that the stability of oxygen adsorbed on thiourea-modified surface was improved and the amount of surface hydroxyl groups adsorbed on this grain surface was decreased. The thiourea adsorbed on SnO2 grain surface is translated to SO42- after sintered at 600 degrees C. SO42- species stabilize the resistance of the SnO2 sensor. (c) 2005 Elsevier B.V. All rights reserved.