An FTIR study of CO adsorption and oxidation on Au/TiO2 is reported. At 90 K different CO-O-18(2) interactions have been performed, with preadsorbed carbon monoxide or preadsorbed isotopically labeled oxygen, with dry or wet carbon monoxide. From the experimental results it can be inferred that (i) at 90 K carbon monoxide and oxygen are molecularly adsorbed on gold particles, (ii) the reaction occurs if carbon monoxide is preadsorbed tit is almost completely inhibited if oxygen is preadsorbed), and (iii) the reaction occurs more easily in the presence of water. It is proposed that dissociation of oxygen is needed in order to observe the oxidation reaction.