Small gas-phase gold cluster cations are essentially inert toward molecular oxygen. Preadsorption of molecular hydrogen, however, is found to cooperatively activate the binding of O-2 to even-size Au-x(+) (x = 2, 4, 6) clusters. Measured temperature- and reaction-time-dependent ion intensities, obtained by ion trap mass spectrometry, in conjunction with first-principles density-functional theory calculations, reveal promotion and activation of molecular oxygen by preadsorbed hydrogen. These processes lead to the formation of a hydroperoxo intermediate on Au-4(+) and Au-6(+) and culminate in the dissociation of O-2 via the release of H2O. Langmuir-Hinshelwood reaction mechanisms involving the coadsorption of both of the reactant molecules are discussed for both cluster sizes, and an alternative Eley-Rideal mechanism involving hydrogen molecules adsorbed on a Au-6(+) cluster reacting with an impinging gaseous oxygen molecule is analyzed. Structural fluctionality of the gold hexamer cation, induced by the adsorption of hydrogen molecules, and resulting in structural isomerization from a ground-state triangular structure to an incomplete hexagonal one, is theoretically predicted. Bonding of H-2 on cationic gold clusters is. shown to involve charge transfer to the clusters. This serves to promote the bonding of coadsorbed oxygen through occupation of the antibonding 2 pi* orbitals, resulting in excess electronic charge accumulation on the adsorbed molecule and weakening of the O-O bond. The theoretical results for hydrogen saturation coverages and reaction characteristics between the coadsorbed hydrogen and oxygen molecules are found to agree with the experimental findings. The joint investigations provide insights regarding hydrogen and oxygen cooperative adsorption effects and consequent reaction mechanisms.