Ethanol (EtOH) and methyl tert-butyl ether (MTBE) are finding increasing use as fuel additives because of governmental mandates, and the relative merits of each have therefore become the focus of intense debate. The ultimate fate of fugitive emissions of these species forms one aspect of this controversy, because EtOH has been implicated in reductions in air quality, while MTBE has been linked to human illness. Both species have been shown to photoreact rapidly on fly ash particles in urban atmospheres. This work reports kinetic studies as well as detailed surface and bulk characterizations of fly ash by XRD, XPS, and SEM/EDA. The active phases in fly ash are probably Fe and Ti oxides. Addition of water to the reactant gas in some cases more than doubled the reaction rate of EtOH, but hardly affected that rate of MTBE. To gain insight into the respective roles of Fe, Ti, and alkali metals, kinetics were also measured over alkali-modified TiO2 (anatase). Addition of Na or Ca oxides shifted the reaction product of EtOH from total oxidation (CO2) to partial oxidation (acetaldehyde).