Hydrocarbon clusters formed by the thermal decomposition of propylene on Pt(111) were rehydrogenated or oxidized with nanometer spatial resolution using the platinum tip of a scanning tunneling microscope (STM) at 300 K in atmospheric pressures of H-2 or O-2 The reaction rate shows a strong dependence on the oxygen or hydrogen pressures and on the tip-surface separation. The reaction stops when the Pt tip becomes contaminated with carbon, after the removal of similar to 10(7)-10(8) carbon atoms, but can be regenerated by removing material from the tip by application of a voltage pulse. Dissociative adsorption of H-2 and O-2 on the tip, followed by transfer of atoms to the surface is the proposed mechanism of these tip-catalyzed reactions.