The reaction CH3CHOHCH3 + 1/2O(2) - 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, P-reaction = P-acetone + P-propene, is 0 03 P-acetone is less than 0.01. In the presence of light with hv > 3.2 eV, P-propene similar to0, and P-acetone = 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, P-reaction = 0.13, and for the photocatalytic pathway, P-reaction = P-acetone = 0.03. Whereas the (110) surface shows a high selectivity for the photocatalytic pathway, the (100) surface shows a high selectivity for the thermal pathway. We discuss this difference in terms of the different adsorption site geometry on these surfaces. For the (100) surface sites, bridging oxygens are closer to Ti4+ binding sites than on the (110) plane. This facilitates proton transfer, which is necessary for the thermal reaction pathway. The photocatalytic pathway is dominant at the (110) surface sites because hydrogen abstraction proceeds more rapidly from the cation resulting from hole trapping than through proton transfer from the neutral molecule.