An FTIR and quadrupole mass study of CO adsorption and oxidation with O-16(2) and O-18(2) on Au/ZnO catalysis is presented. The experimental results indicate that : (i) CO is activated by gold in two molecular forms, a linear carbonyl species bonded at terrace Au sites and a carbonyl species bonded to Au peripheral sites; (ii) a band related to CO adsorbed on Au oxidized sites and a scrambling reaction between CO and O-18(2) indicate that oxygen is also activated on gold sites. The oxygen adsorbed on gold is probably strongly basic, as is the oxygen adsorbed on silver and on copper, and it can easily oxidize CO to CO2.