We have studied the catalytic effect of adsorbed cesium on the initial sticking coefficient s0 of oxygen on Si(100) at room temperature. Two different kinds of cesium sites were observed. s0 was found to increase linearly with the coverage of each kind of cesium site. The data show that the effect of cesium is local, but s0 does not correlate with the Cs dipole moment. An oxygen-cesium bond is formed in the reaction and cesium is passivated in this process, leading to an exponential decrease of the sticking coefficient with the oxygen coverage. © 1990 The American Physical Society.