Secondary-ion mass spectrometry (SIMS) and electron-stimulated desorption (ESD) have been used to characterize the composition of antimony-doped tin oxide films before and after a caustic treatment and before and after deposition of platinum. The impurities were identified qualitatively using dynamic SIMS and found to be Li, C, N, Na, Mg, Al, Si, S, K, Ca, Cr, V, Fe, Mn, Ni, Ti, and Zn. Both SIMS and ESD demonstrated that hydrogen is a major constituent of these films. Evidence is presented that the caustic treatment hydroxylates the surface and that Sb is segregated at the surface. It appears that platinum desorbs by electron stimulation probably in the form of an oxide species. This may prove useful in probing the composition of supported-metal crystallites.