Structure, reducibility and catalytic activity in the oxidation of CO were studied for Co-modified ZSM-5 zeolites (Co/Al = 0.1-0.9) prepared via impregnation, solid-state and wet ion exchange. According to TEM, XPS, DRIFTS, UV-vis-DRS, H-2-TPR and in situ CO-reduction studies the synthesis method and the Co content affect the surface distribution of Co as Co2+ ions, cationic oxo-complexes and highly dispersed CoxOy species. The oxidation of CO by oxygen was studied at 50-450 degrees C in pulse and flow regimes at the 2:1 and 1:1 CO:O-2 ratios. Co-zeolites prepared by impregnation were the most active in the oxidation of CO. It was proposed that oxo cations [CoxOy ](n+) (n = 1, 2) with Co(III) prevailing in their structure play a crucial role in CO adsorption and catalytic oxidation. The new absorption band at 2215 cm(-1)attributed to CO adsorbed on the Co(III) located in oxo cations was found. These oxo-cations contain a mobile oxygen atoms and, thus, can be easily reduced to the catalytically active Co3+/Co(2+)and Co2+/Co+ pairs stabilized in zeolites. In contrast to the supported catalysts based on Co3O4, the activity of Co-ZSM-5 increased during the reaction, and the light-off T50 drifted to 120 degrees C.