We have monitored the change in work function during the initial oxidation of discharge‐produced amorphous Si(H) and correlated this change with the amount of adsorbed oxygen as measured by Auger spectroscopy. With exposure to oxygen, the work function first increases (explained in terms of a dipole layer on the surface) and then decreases (explained in terms of penetration of oxygen below the surface). The explanation appears to be confirmed by similar measurements on the three primary crystallographic faces of single‐crystal Si. Initial sticking coefficient and dipole strength of oxygen adsorbed on amorphous Si are given.