The influence of the Si/Al ratio, of the nature of the T-atom and of the pore size on the acidic strength of Bronsted sites in zeolites has been investigated using changes of the vibrational properties of Bronsted OH(OD) groups and a shift change of Bronsted protons in nuclear magnetic resonance upon adsorption of weak bases. Deuterated acetonitrile and trichloro-acetonitrile have been chosen to probe the acidic strengths of ZSM-5, FeZSM-5, mordenite and zeolite Y, which are often used as catalysts. From the results of the FTIR and H-1 MAS NMR studies it can be concluded that the chemical composition of the lattice dominates the acidic strength of the Bronsted sites in zeolites. Differences in structure or pore size play a much smaller role.