Scanning tunneling microscopy (STM) is demonstrated to be a powerful tool to characterize adsorption and reaction on oxide surfaces by imaging molecular adsorbates and reactive intermediates. The molecules were used to probe surface structure and to study surface reactivity spatially at the atomic level. Results for three systems are presented: alcohol adsorption on WO3 (0 0 1), carboxylates on the anatase polymorph of TiO2, and propene adsorption on a PdO monolayer on Pd(1 0 0). When the alcohols were exposed to the WO3(0 0 1)-c(2 x 2) surface at room temperature the molecules could not be imaged. Heating the surface to temperatures above a water desorption peak associated with alcohol deprotonation, however, allowed 1-propoxide to be imaged. The images reveal that the alkoxide has no preference for defects, rather it binds to W6+ ions exposed on the fully oxidized c(2 x 2) surface. Temperature-programmed desorption revealed that alkoxides at these sites undergo only dehydration reactions. To probe the structure of the unusual (1 x 4) reconstruction on anatase (0 0 1), formic and acetic acid adsorption were used. Following dissociative adsorption, both formate and acetate adsorb solely centered atop the bright rows that define the surface reconstruction, and the molecules are always at least two lattice constants apart. This result may be attributed to carboxylates bridge-bonded to Ti atoms at the center of the bright rows. This finding eliminates several suggested models of the reconstruction and suggests that a recently proposed ad-molecule model is a good representation of the surface structure. Propene was observed to initially randomly adsorb on the PdO monolayer. At higher coverages, however, the adsorbates cluster, disrupting the surface structure and causing the adsorption rate adjacent to the clusters to increase. Temperature-programmed reaction revealed that once propene adsorbs, the oxide monolayer catalyzes its oxidation at lower temperatures than metallic Pd, but that the propene sticking coefficient on the ordered oxide layer is a factor of 5 lower. (C) 2003 Elsevier B.V. All rights reserved.