The photocatalytic production of acetone from 2-propanol and O-2 has been investigated on the TiO2(110) surface using molecular beam techniques. We have also studied the thermal reaction of 2-propanol with O-2 on this surface in the absence of light. We find that, in the absence of light, the probability of producing propene is greater than that for producing acetone, but the total probability for these reactions is less than 0.03 for a single collision of the incident molecule with the surface. This probability increases to 0.15 if band gap radiation is present on the surface, with acetone production being the dominant reaction channel. The maximum reaction probability is reached on a surface that has been preannealed in a vacuum to create oxygen vacancies. Fully oxidizing the surface reduces the probability to 0.08. The steady-state reaction yield is maximized at a surface temperature of 350 K and decreases to low values at 250 and 600 K. The yield is limited by desorption of acetone below 300 K and by the decrease in the surface coverage of reactants above 400 K. The absence of thermal reactivity and the significant photochemical reactivity is attributed to free-radical reactions initiated through electron trapping by adsorbed molecular oxygen. Our results suggest that the reaction proceeds through a mechanism in which holes are trapped by undissociated 2-propanol molecules.