Ni-ZSM-5 and Ni/SiO2 samples have been characterized by the IR spectra of adsorbed and coadsorbed NO and CO. Adsorption of CO at room temperature on Ni/SiO2 leads to formation of Ni2+-CO species (v(CO) at 2192 cm(-1)) that are easily destroyed by evacuation. The carbonyls formed on Ni-ZSM-5 after CO adsorption are two types and characterized by bands at 2220 and 2212 cm(-1). In line with this high frequency, both species are much more stable than the Ni2+-CO species on Ni/SiO2 and are highly resistant toward evacuation. Reduction of Ni-ZSM-5 with CO results in the appearance of Nil ions. With CO these ions form Ni+(CO)(2) dicarbonyls (v(s) at 2136 cm(-1) and v(as) at 2092 cm(-1)), which lose one of their CO ligands during evacuation and are thus converted into Ni+-CO linear species (2109 cm(-1)). In contrast, no Nil ions are produced after reduction of the Ni/SiO2 sample. NO adsorption on Ni/SiO2 results in formation of Ni2+-NO species (1870 cm(-1)) which are converted, under NO equilibrium pressure, into Ni2+(NO)(2) dinitrosyls (v(s) at 1870 cm(-1) and v(as) at 1842 cm(-1)). Similar species are formed on Ni-ZSM-5: the mononitrosyls are characterized by bands at 1905-1895 cm(-1), whereas the bands typical of dinitrosyls are at ca. 1900 cm(-1) and 1874-1862 cm(-1). After evacuation only mononitrosyls are present on the sample. With the Ni/SiO2 sample NO replaces preadsorbed CO. However, the situation with Ni-ZSM-5 is quite different. Here, mixed Ni2+(CO)(NO) species are clearly detected (v(CO) at 2147 cm(-1) and v(NO) at 1863 cm(-1)). In these complexes the bond between Ni2+ and the ligands is weakened and, as a result, CO is easily removed by evacuation. The reasons for the different properties of nickel cations in different matrixes are discussed. It is proposed that the low coordination number of cations in ZSM-5 is the main reason for the formation of mixed complexes.