The transient kinetics of the removal (titration) of oxygen adatoms from a cesium-dosed Cu(110) surface by reaction with H2 gas to produce H2O were studied in a system which combined ultrahigh vacuum surface analysis and medium-pressure gas exposures in an attached microreactor. Surface temperatures of ∼ 480-700 K and H2 pressures of 10-2-10-3 Torr were used. An inert gas (N2) was added to the microreactor at pressures up to 50 Torr to vary the temperature of the H2 gas molecules hitting the Cu surface from the wall temperature (300 K) up to the surface temperature. With these molecules at the surface temperature, the titration reaction showed an apparent activation energy of 18 ± 2 kcal/mol, a preexponential factor of ∼ 5 O atoms removed per H2 collision with the surface, and an order in H2 pressure of 0.70±0.15. (These values are appropriate for θCs ≅ 0.2 and 0.15 ≤ θo≤ 0.8, where the rate was nearly independent of θo.) These values are similar to those obtained on Cu(110) without Cs present, which suggests that the same elementary step dominates the kinetics here: the dissociative adsorption of H 2. Consistent with this is the strong influence of H2 gas temperature on the rate. © 1991.