High-resolution photoelectron spectroscopy using synchrotron radiation was applied for monitoring in real time the oxidation kinetics on the Ti(0001) surface at 405 degrees C with dry O2 gas. The time evolution of O 1s photoelectron intensity showed a linear uptake curve up to approx. 90 L followed by a sudden saturation up to approx. 160 L and then a restart of the linear increase, indicating that O2 adsorption obeys a zero-order reaction scheme before and after the saturation. Corresponding to the first linear uptake and saturation, the surface core level shift (SCLS) component of Ti 2p decreased predominantly and disappeared completely, and appeared again after the saturation and remained persistently during TiO2 growth. Thus the zero-order reaction of O2 adsorption on the Ti(0001) surface at 405 degrees C is concerned with the metallic Ti layer on the outermost surface.