The effect of oxygen adsorption on the (100) tungsten surface has been investigated in a thermionic electron emissionmicroscope and by means of contact potential measurements (Kelvin method). Surface contamination was closely controlled. The measurements indicate that the work function versus oxygen pressure curves agree with those of Batzies. The oxygen desorption energy decreases with increasing oxygen coverage. The initial sticking coefficient S 0 , slope (dΔφ/dn) 0 and dipole moment μ D deduced from the experimental results are S 0 =0.17, d(Δφ/dn) 0 =4.3×10 −15 eV at −1 cm 2 , μ D =2.2 D .