SnO2 microspheres were synthesized by a chemical route, heating the mixed Sn2+ and sulfuric acid solution in the presence of pressurized oxygen at 900 degrees C in a designed calorimetric pump. Phase analysis was carried out by X-ray diffraction (XRD) and the results confirmed the SnO2 microspheres as a single-phase tetragonal structure. Scanning electron microscopy (SEM) images indicated that these microspheres with average diameters of 0.9 mu m were composed of SnO2 nanoparticles with a diameter of about 4.7 nm and a crystallite size of 3.7 nm, as observed by transmission electron microscope (TEM) and calculated by Scherrer's equation from diffractograms, respectively. The formation mechanism of microspheres composed of SnO2 nanoparticles and the influence of changes in processing are proposed and explained. (c) 2009 Elsevier B.V. All rights reserved.