A series of antimony-doped tin oxide (ATO), Sn(Sb)O-2, nanophase catalysts (containing 43 mol% Sb calcined at temperatures of 250, 600, and 1000degreesC, respectively) have been studied by correlated electron energy-loss spectroscopy (EELS) and Z-contrast imaging in a scanning transmission electron microscope (STEM) and by high-resolution electron microscopy (HREM). As reference materials, pure SnO2, Sb2O3, and Sb2O5 are also analyzed. Results show that the particle size of these ATO catalysts increases with the calcination temperature. At low calcination temperatures, the catalyst contains crystalline nanoparticles surrounded by amorphous material, while the nanoparticles become well crystallized when calcined at higher temperatures. EEL spectra acquired from these ATO catalysts are qualitatively interpreted using 'fingerprints' from the reference materials. Results suggest that at low calcination temperatures (250 degreesC), Sb exists in the pentavalent state Sb (V) either incorporated into the SnO2 lattice or as an amorphous layer surrounding the SnO2 crystals. At higher temperatures (greater than or equal to600degreesC), Sb segregation occurs as trivalent Sb (III) at the surface and Sb (V) in the core of the majority of the nanoparticles. However, it should be noted that nearly no Sb was detected in some particles. The surface Sb (III) ions are proposed as active sites in the ATO catalysts.