Photoemission energy distribution curves are presented for adsorbed layers of O on W(110) and CO on W(100) surfaces. The electronic energy levels of adsorbed oxygen initially form a 2.0 eV-wide peak at −6.4 eV below the Fermi level which shifts to −6.0 eV and broadens to 2.8 eV with increasing coverage. Provision for simultaneous LEED, Auger spectroscopy, and flash desorption enable the structure and coverage of the adsorbed layers to be determined. β phase CO adsorbed in either the (1×1) or C(2×2) structures on W(100) produces a photoemission peak at −6.0 eV with a weaker peak at −3.5 eV. Adsorption of CO in the a phase gives rise to a 2.2 eV-wide peak at −8.9 eV which is identified as originating from levels essentially the same as those of molecular CO.