Al 2O 3-supported osmium catalysts were prepared from Os 3(CO) 12 and from K 2[OsO 2(OH) 4]. The cluster-derived catalyst was modified with potassium by an additional impregnation with K 2CO 3. The materials were physically characterized by carbonyl infrared spectroscopy, temperature-programmed reduction and transmission electron microscopy. Isolated surface complexes were the majority species in the Os 3-derived catalysts, their oxidation state being +2 after thermal treatment while Os(0) carbonyl species were formed after H 2 reduction. These did not aggregate to metal particles due to the presence of the protecting CO ligands. Catalysts prepared from the potassium osmate compound behaved differently for different loadings. At low loading (1.6 wt.% Os) Os 2+ species were highly stabilized even after reducing treatments. CO turned out to be a more efficient reducing agent than H 2. In contrast, at high loading (4.2 wt.% Os) metal aggregation was observed after H 2 reduction, with average metal particle diameters of 1.8 nm. The major effect of potassium in these materials seemed to be blocking of the osmium species for CO adsorption at low temperatures (⩽300K). The formation of bicarbonate, carbonate and formate species was also detected during the adsorption of CO. These species may be involved as intermediates in catalytic reactions such as water-gas shift. Possible formation routes of these species and the role of potassium are discussed.