The initial stages Of O-2 oxidation of H-passivated flat and vicinal Si(1 1 1) surfaces are investigated by monitoring the Si-H stretch vibrations with infrared absorption spectroscopy. We find that the incorporation of oxygen into silicon is activated (1.66 +/-0.10 CV on flat surfaces), involving a multistep process. Oxygen molecules are incorporated into Si-Si bonds without removing surface hydrogen and this process is facilitated at steps.