The interaction of oxygen with the stable Ir{1(0) over bar 0}-(1x5) and the metastable (1x1) surfaces has been studied using supersonic molecular beams in the surface temperature range 200-1080 K. Starting from the clean (1x5) substrate, the adsorption kinetics are dominated by the adsorbate-induced lifting of the reconstruction. The formation of(1x1) islands occurs between two limiting oxygen surface coverages, as confirmed by helium scattering and low-energy electron diffraction (LEED) measurements. Two distinct temperature regimes are observed in the sticking probability measurements; between 350 and 600 K the local oxygen coverage on the (1x1) phase is about 0.28 monolayers (ML) during the prevailing phase transformation, whereas it is 0.20 ML in the temperature range 700-900 K. This ＇＇biphasic＇＇ behavior is explained by the enhancement of surface diffusion of adsorbed oxygen atoms at sample temperatures above 650 K and has been investigated further using thermal energy atom scattering (TEAS). In contrast to the(1x5) phase, TEAS measurements show that random adsorption of O-2 takes place on the clean metastable (1x1) surface. At 1080 K a pronounced flux dependence of the sticking probability is observed due to a nonlinear growth law for the formation of (1x1) islands, r=c(B theta(O)(1X5))(4.5). Thermal desorption measurements accompanied by LEED show that the desorption rate is strongly influenced by the (1x1) to (1x5) surface phase transition; repulsive lateral interactions exist between adsorbed oxygen atoms on the (1x1) substrate. We present a mathematical model which takes these effects into account in reproducing the salient features of the temperature programmed desorption (TPD) spectra. Sticking probability, TEAS, and TPD data are all consistent with a defect concentration of 0.03 ML on the clean (1x5) surface annealed at 1400 Kt (C) 1998 American Institute of Physics. [S0021-9606(98)01546-3].