The chemical nature of Co2+ ion-exchanged ZSM-5 catalyst has been studied by electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and diffuse-reflectance Fourier-transform infrared spectroscopy (DRIFT). EPR and in-situ XPS investigations show that the oxygen coordination geometry around the Co2+ at intersections or in channels of ZSM-5 does not have high symmetry and that Co2+ cations occupy at least two different sites in the zeolitic pores. The oxidation state of Co2+ cations in ZSM-5 zeolites remains unchanged after being treated in oxygen at 450 degrees C, while the locations and coordination of the Co2+ cations may have been altered. In-situ DRIFT studies of Co-ZSM-5 illustrate a strong interaction of Co2+ cations with coordinated molecular oxygen species, indicating a modification of the catalytically active sites in Co-ZSM-5 by oxygen or NO under the simulated exhaust condition.