The adsorption of ethene and propene on titania and titania-supported rhodium has been studied, observing a continuous adsorption even after 24 h. The samples have been reduced at 623 or 773 K, but no difference was observed, whichever the reduction temperature or the presence of rhodium, on the amount of gas adsorbed per unit of surface area of the solid. A study of the polymerization has been done following the “equal reactivity hypothesis”, and it has been concluded that the role played by the rhodium particles is to facilitate adsorption of the gaseous molecule, that is then transfered to the surface of the support, where it polymerizes.