The influence of the state of iron on the catalytic properties of Fe(x)SiBEA zeolite in the oxidation of CO was investigated. Various Fe(x)SiBEA zeolites (x = 0.6, 1.0, 4.0 and 10 wt%) were prepared by a two-step post synthesis method. The presence of Fe(III) in tetrahedral coordination for low Fe content and tetrahedral and octahedral coordination for high Fe content is evidenced by diffuse reflectance UV-Vis and TPR. FTIR investigations of pyridine adsorption show that Bronsted and Lewis acidic sites are formed upon incorporation of iron in SiBEA zeolite. The amount of both acidic sites increases with Fe content. The catalytic activity of FexSiBEA in the oxidation of CO strongly depends on the state of iron in BEA structure. Fe(0.6)SiBEA and Fe(1.0)SiBEA containing mainly framework tetrahedral Fe(III) ions are more active than Fe(10)SiBEA containing mainly extra-framework FeO(x) oligomers. The most active catalyst is Red-Fe(1.0)SiBEA obtained after reduction of Fe(1.0)SiBEA at 760 degrees C in flowing H(2). It shows almost 100% CO conversion at room temperature. (C) 2011 Elsevier B.V. All rights reserved.