We show that the dissociation probability of O-2 on the reconstructed, Au(111)-herringbone surface is dramatically increased by the presence of some atomic oxygen on the surface. Specifically, at 400 K the dissociation probability Of 02 on oxygen precovered Au(111) is on the order of 10(-3), whereas there is no measurable dissociation on clean Au(111), establishing an upper bound for the dissociation probability of 10(-6). Atomic oxygen was deposited on the clean reconstructed Au(111)-herringbone surface using electron bombardment of condensed NO2 at 100 K. The dissociation probability for dioxygen was measured by exposing the surface to O-18(2). Temperature programmed desorption (TPD) was used to quantify the amount of oxygen dissociation and to study the stability of the oxygen in all cases. Oxygen desorbs as O-2 in a peak centered at 550 K with pseudo-first-order kinetics; i.e., the desorption peak does not shift with coverage. Our interpretation is that the coverage dependence of the activation energy for dissociation (Delta E-dis) and/or preexponential factor (nu(d)) may be responsible for the unusual desorption kinetics, implying a possible energy barrier for O-2 dissociation on Au(111). These results are discussed in the context of Au oxidation chemistry and the relationship to supported Au nanoparticles.