The oxidation of sulfur dioxide to sulfur trioxide over supported vanadium oxide catalysts occurs as both a primary and secondary reaction in many industrial processes, e.g., the manufacture of sulfuric acid, the selective catalytic reduction of NO, with ammonia and the regeneration of petroleum refinery cracking catalysts. This paper discusses the fundamental information currently available concerning the molecular structure and sulfur dioxide oxidation reactivity of surface vanadia species on oxide supports. Comparison of the molecular structure and reactivity information provides new fundamental insights on the following topics related to the catalytic properties of surface vanadia species during the sulfur dioxide oxidation reaction: 1. role of terminal V=O, bridging V-O-V and bridging V-O-support bonds, 2. number of surface vanadia sites required to perform SO2 oxidation, 3. influence of metal oxide additives, 4. generation and influence of the surface sulfate overlayer, 5. effect of surface acidity on the reaction turnover frequency, 6. competitive adsorption between SO2 and SO3 and 7. reaction kinetics.