Surface chemical analysis of Zr-O/W system was performed at two different sample temperatures, room temperature and ∼1600 K, by using Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). Depositing Zr onto a polycrystalline W-substrate the surface was exposed to oxygen. AES and ISS measurements have revealed that the oxygen adsorbed on the Zr/W surface at room temperature diffuses into Zr/W at ∼1600 K, keeping a major part of Zr remained on the outermost atomic layer. This result strongly supports the model proposed by Danielson and Swanson for Zr-O/W (100) system that the oxygen adsorbed on the Zr/W (100) surface at room temperature diffuses into the Zr/W (100) as a composite Zr-O at high temperature and this results in considerable decrease of work function, leading the Zr-0/W (100) system to an electron source of high brightness.