This paper reports the first use of CO probe molecules in an IR investigation of the redox behaviour of alumina supported rhenium. After O-2 treatment the rhenium surface species are coordinatively saturated and do not allow the adsorption of CO. Treatment with H-2 at 823 K leads to the complete reduction of all rhenium species to rhenium metal. However, partially reduced rhenium Rex+ (where 0 < x < 7) species can be generated through treatment with CO at 333 K, or similarly by exposure to isobutene at the same temperature. These results provide further evidence that partially reduced rhenium species are involved in the metathesis of alkenes. (C) 2003 Elsevier B.V. All rights reserved.