The chapter discusses whether the high-frequency band of the C= N vibration is really due to the species formed as a result of acetonitrile interaction with the products of propene adsorption or should it belong to the nitrile molecules coordinated to A1³⁺ Lewis acid sites. It is known for the complexes of acetonitrile with BHal₃ compounds that the C=N stretching fundamental band reveals small but detectable shifts from about 1 up to 3 cm⁻¹ after ¹⁰B by ¹¹B substitution. This means that upon using isotopically substituted propene, a shift of the nitrile band should be expected due to the strong interaction of the C= N group with carbocations. For this purpose, in the study described in the chapter the infrared (IR) spectra of CD₃CN adsorbed on HZSM-5 zeolite pretreated with both normal propene and propene-d₆ were investigated at 300 K. The observed sensitivity of the CN frequency to isotopic substitution in propene leads to the conclusion that the nitrile C=N group of the resulting product is really bound to the positively charged C atom of propene but not to the Al³⁺ ion of the zeolite framework.