In the presence of low-intensity UV lights, lattice oxygen is extracted from the TiO2 surface at room temperature by adsorbed formic acid. Transient photocatalytic oxidation (PCO) and decomposition (PCD) of formic acid to CO2 and H2O were combined with interrupted reaction and temperature-programmed desorption to directly show that lattice oxygen is the oxidizing species during PCD, whereas adsorbed oxygen oxidizes organics during PCO. The rates of lattice oxygen extraction and diffusion of lattice oxygen from the TiO2 bulk to the surface to replenish the extracted oxygen were measured. The initial rate of decomposition (oxidation) of formic acid in the absence of gas-phase O-2 is one-seventh the rate in 3% O-2. Lattice oxygen vacancies are readily replenished by O-2 in the dark at room temperature, but H2O does not re-oxidize them, even during UV exposure. Surface diffusion of formic acid to active sites does not limit photocatalytic reaction. Lattice oxygen extraction causes slow deactivation of TiO2.