General techniques for the partial oxidation of alkenes by molecular oxygen are a goal for surface science and catalysis research as they may lead to more efficient and environmentally friendly industrial processes. To better understand the thermal surface chemistry of metal oxides toward alkene partial oxidation, the interactions of isobutene and propene on TiO2(110) were studied using temperature-programmed desorption (TPD). Isobutene was found to adsorb and desorb molecularly below 250 K on the clean surface. With exposure to oxygen (> 1000 L) and unknown quantities of water (< 10 L), isobutene monolayers on TiO2(110) react to form products that include methacrolein and isobutanal, as well as a third product that has a C4H8O stoichiometry. We tentatively assign this species to 2,2-dimethyloxirane (isobutene oxide). Structural conservation within this family of products points to a common surface intermediate that we propose results from the addition of O from a hydrogen peroxo (HOO) species to the C=C bond of isobutene. This hydrogen peroxo (HOO) species forms from the reaction of physisorbed water and oxygen, assisted by partial charge transfer from the TiO2(110) substrate. Initial studies reveal a similar reaction pathway for the partial oxidation of propene on TiO2(110), yielding acetone and propanal. This work suggests that TiO2 surface sites on supported Au/TiO2 catalysts are active for partial oxidation of alkenes. (c) 2005 Elsevier Inc. All rights reserved.