Adsorption of CO on Ni-ZSM-5 reveals the existence of two kinds of Ni2+ ions, with the respective stretching frequencies being at 2220 and 2212 cm(-1). The carbonyl complexes are resistant to evacuation at room temperature, which is explained by the high electrophilicity of cations in a ZSM matrix. At 85 K and in the presence of CO, part of the Ni2+-CO species are converted into Ni2+(CO)(2) dicarbonyls (2204 cm(-1)) which are characterized by a very low stability and easily lose one of their CO ligands. Interaction of Ni-ZSM-5 with CO at 673-773 K results in a preferential reduction of the Ni2+ ions which form the carbonyls characterized by the band at 2220 cm(-1). CO adsorption at room temperature on the reduced sample leads to the formation of Ni+(CO)(2) gem dicarbonyls (v, at 2136 cm(-1) and v, at 2092 cm(-1)) which are converted, during evacuation, to linear Ni+-CO species (2109 cm-1). The dicarbonyl structure is proved by (CO)-C-12-(CO)-C-13 coadsorption, and the mixed Ni+((CO)-C-12)((CO)-C-13) species are characterized by v((CO)-C-12) at 2123 cm(-1) and v((CO)-C-13) at 2058 cm(-1). At low temperature, a third CO, molecule is coordinated to the Ni+ cations, thus producing tricarbonyls (2156, 2124, and 2109 cm(-1)). The latter species lose one of their CO, ligands during evacuation at 85 K and are converted back into dicarbonyls. The Ni+ sites are easily oxidized in the presence Of O-2. However, the Ni2+ ions produced differ in properties from the initially deposited cations and are easily reduced to Ni+ even at ambient temperature.