Bimetallic FeIr/SiO 2 CO hydrogenation catalysts have been studied by temperature-programmed reduction, EXAFS, Mössbauer spectroscopy, and previously also by infrared spectroscopy of adsorbed CO. We concentrate on the structure of a freshly reduced sample, which is selective for the formation of hydrocarbons from CO + 3H 2 , and a catalyst activated during 48 h of reaction, which produces mainly methanol. The fresh sample contains iron oxide and bimetallic FeIr particles with an iron-rich surface after reduction in H 2 at 450 • C. During CO hydrogenation, a part of the oxidic iron reduces and forms, together with some of the iron that is initially alloyed an iron carbide. CO adsorption on activated FeIr/SiO 2 is considerably weaker than on the freshly reduced catalyst, while the activated catalyst also possesses a much higher hydrogenation activity than the initially reduced system does. Structure models for the catalyst after reduction and during steady state CO hydrogenation at high pressure are discussed.