The growth and characterization of sulfuric acid (H2S04) films in ultrahigh vacuum (UHV) is described. The films were synthesized in situ by co-condensing SO3 and H20 onto a single crystal metal surface at 100 K and carefully annealing the mixtures above 160 K. Films were typically 30-50 monolayers thick and were characterized using Fourier transform infrared reflection absorption spectroscopy (FTIRAS), temperatureprogrammed desorption (TPD), and X-ray photoelectron spectroscopy ( X P S ) . The FTIRAS, TPD, and XPS spectra show that solid homogeneous mixtures of varying water content can be prepared, from approximately 10 mol % H2S04 to nearly pure H2S04. The nearly pure films can be prepared in crystalline and amorphous forms, depending on the annealing procedure employed. Some surface chemical properties of the pure H2S04 films were investigated. 2-Propanol is readily adsorbed and absorbed by H2S04, with extensive dehydration to water and propene occurring at relatively low temperatures (<320 K). Pure solid HzSO4 does not adsorb or absorb HCl at 100 K, in contrast to ice, which readily adsorbs HCl at 100 K, but in striking similarity to concentrated, liquid H2S04 at room temperature.