Ethanol and methyl tert-butyl ether (MTBE) are finding increasing use as fuel additives in the U.S. 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 ethanol has been implicated in reductions in air quality, while MTBE has been linked to human illness. Both species are known to react homogeneously with photogenerated OH radicals. Here we show that both can also photoreact on fly ash and related particulates present ubiquitously in urban tropospheres. The heterogeneous reactions involving MTBE ultimately form formaldehyde and acetone at rates comparable to purely homogeneous pathways, demonstrating for the first time the existence of a significant catalytic process on tropospheric particulates having widespread spatial distribution. Oxides of Ti and Fe drive the reaction.