We report a facile, low-temperature reaction between condensed-phase NO2 and H2O that leads ultimately to the formation of a large concentration (theta(o) = 0.42 monolayers) of adsorbed oxygen adatoms on a Au(111) surface. This reaction represents a novel route to generate surface oxygen on Au that can be utilized in fundamental studies of oxidation catalysis over Au such as low-temperature CO oxidation and the operation of Au converter tubes used in redox chemiluminescence detectors for atmospheric NOx. Also, this reaction may be relevant to heterogeneous processes occurring on ice particles in stratospheric clouds. This reaction is not specifically catalyzed by the Au(111) surface or by the defects present on the Au surface, and so this reaction may have some general utility for facile oxidation of unreactive surfaces.