The reaction CH3CHOHCH3 + 1/2O2 → CH3COCH3 + H2O has been studied on the (110) and (100) planes of TiO2 using molecular beam techniques. We find that the reaction can proceed through thermally activated and photocatalytic pathways to produce acetone and propene. On the (110) surface, in the absence of light the total reaction probability for an incident 2-propanol molecule, Preaction = Pacetone + Ppropene, is 0.03. Pacetone is less than 0.01. In the presence of light with hυ > 3.2 eV, Ppropene ∼ 0, and Pacetone = 0.15 and the reaction proceeds readily for T < 180 K, indicating that it is not thermally activated. On the (100) surface, we also observe thermally activated and photocatalytic pathways. However, the branching ratio is very different than that on the (110) surface. We observe that, for the thermally activated channel, Preaction = 0.13, and for the photocatalytic pathway, Preaction = Pacetone = 0.03. Whereas the (110) surface shows a high selectivity for the photocatalytic pathway, the (100) surf...