The main idea of the present work is the creation of new types of redox active sites by using zeolite as a matrix. Structure and catalytic activity in CO oxidation were studied for catalysts based on CoÀ Ce-modified ZSM-5 synthesized by sequential incipient wetness impregnation. Synergy of action of the two additives is observed. The most active is the catalyst with the ratio Co : Ce = 3 : 1. According to TEM, XPS, UV-vis-DRS, DRIFTS and in situ CO-reduction studies, the interaction of metals occurs in the channels of the zeolite with the formation of previously unknown joint structures in which Co III and Ce III are stabilized. Apparently, these structures are represented by mixed CoÀ Ce oxo-cations [Co x CeO y ] n + . A new absorption band at 2179 cm À 1 appearing in the DRIFT spectra of adsorbed CO is associated with mixed species. The catalytic activity in the oxidation of CO correlates with amount of these structures.[a] M.