Quantitative IR studies were realized in order to determine the concentration of and in two zeo- lites NiZSM-5 differing in Si/Al and nickel content. The NiZSM-5 zeolite of Si/Al = 11.5 contained most of in exchange positions whereas another one, of Si/Al = 40 contained most of in the form of oxides. Also the ions in NiZSM-5 reduced by CO were investigated. The experimental procedure of quantita- tive studies was elaborated and the condition of stoichiometric reaction between probe molecule (CO or NO) and sorption site ( or ) was fulfilled: each CO or NO molecule introduced into the IR cell reacted with one forming or adducts. The procedure was also elaborated for quan- titative studies of concentration of in zeolite reduced. The absorption coefficients of IR bands of , and at 2212, 1898, and 2109 , respectively were determined, and finally the concentrations of and were calculated. In the zeolite of Si/Al = 11.5 the concentration of in exchange positions determined by our IR studies was nearly the same as that determined by chemical analysis. The values of the absorption coefficients turned out to be independent on the probe molecule (CO or NO). In the zeolite of Si/Al = 40, containing the most of in the form of oxides, the concentration of found by IR was 66% of concentration of Ni determined by chemical analysis demonstrating that dispersion of was low and some of was hidden inside oxide clusters being inaccessible to probe molecules. In reduced NiZSM-5 of Si/Al = 11.5 and 40 the concentration of determined by IR studies was nearly the same as concentration of Ni determined by chemical analysis. In NiZSM-5 of Si/Al = 40 the dispersion of Ni increased distinctly upon the reduction, and all cations became accessible to probe molecules or reactants.