Adsorption and reaction of water on the clean and oxygen modified Ir(1 1 1) single crystal surfaces have been studied using temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS) techniques under ultrahigh vacuum (UHV) conditions. Water dissociates on the clean Ir(1 1 1) surface with a probability (estimated based on production of hydrogen) which decreases from similar to 0.016 to 0.004 +/- 0.0015 with increasing water coverages from 0.34 to 2.59 monolayer. Scattering experiments performed at various surface temperatures in the limit of zero coverage yield water dissociation probabilities in the range of similar to 0.0005-0.012 (300-900 K) with an uncertainty expressed as +/- 20% of the dissociation probability. The apparent activation energy for water dissociation on clean Ir(1 1 1) is estimated to be similar to 170 +/- 5 kJ/mol employing MBRS techniques, which probably cannot be applied to TPD measurements with higher water coverages. We speculate that water dissociation occurs on the defects of the Ir(1 1 1) surface. Using isotopically labeled reactants, a strong interaction between adsorbed water and oxygen was found on Ir(1 1 1), indicated by a new water desorption feature at 235K and scrambled oxygen and water desorption products. (C) 2010 Elsevier B. V. All rights reserved.