The adsorption of hydrogen (deuterium) and oxygen on neutral platinum clusters has been investigated in a cluster beam experiment. The beam passes through two low-pressure reaction cells and the clusters, with and without adsorbed molecules, are detected by laser ionization and mass spectrometry. Both H-2 and O-2 adsorb efficiently on the platinum clusters with only moderate variations with cluster size in the investigated range, i.e., between 7 and 30 atoms. The coadsorption of H-2 and O-2 results in the formation and desorption of H2O, detected as a decreasing number of adsorbed oxygen atoms with an increasing number of collisions with H-2 molecules. The water-formation reaction proceeds efficiently on all investigated clusters with more than seven atoms.