The use of IR spectroscopy of adsorbed CO for determination of gold speciation on supported gold catalysts is reviewed. Different ionic and metallic sites (Au3+, An(+), Au-0, and Au-) can be differentiated on the basis of the frequency and stability of their surface carbonyl species formed after CO adsorption. in addition, the study of CO adsorption and reaction with other gases can provide unique information on the mechanisms of catalytic reactions with participation of CO. Finally, results from complementary techniques such as X-ray absorption spectroscopy, also useful for determination of gold oxidation state, are briefly considered.