The interaction of H2O vapour with clean and oxidized Pb(100) single crystal surfaces has been studied using photoelectron spectroscopy. The clean Pb(100) surface, does not react with H2O vapour at 298 K but physisorbs H2O at a very high sticking probability (? unity) at 80 K. The adlayer is completely desorbed at 150 K, leaving an atomically clean surface. At 80 K, H2O is molecularly adsorbed on oxidized lead surface without decomposition, but on warming to 298 K there is a strong evidence for a significant decomposition. Exposure of the Pb(100) - O surface to water vapour at 298 K leads to stoichiometric transfer in intensity from the oxide region to the hydroxyl region indicating the formation of PbO(OH)2(s). Oxidized Pb(100) single crystal shows no catalytic tendency to decompose ethylene, propylene and acrolein in the temperature range (298-450 K).