FTIR spectroscopy was used to characterise Ru-3(CO)(12) supported by chemical vapour deposition onto several gamma-alumina samples with different dehydration degrees. The metal carbonyl was found to be molecularly physisorbed onto hydrated alumina; it undergoes oxidative addition of a surface OH group onto hydroxylated alumina; it forms Lewis type adducts, via a CO group, with Al3+ ions at the surface of dehydroxylated alumina.