A dried (773 K, Ar, 1 h) NaH-ZSM-5 sample (Si/Al = 20, Na/Al = 0.5) showed two sharp O–H vibrations at 3745 cm −1 (SiOH, terminal OH) and at 3612 cm −1 [Si(OH)Al, bridged OH], a weak, wide band at 3300–3200 cm −1 due to H-bonded OH and three characteristic (T–O–T) vibrations at 1268, 975 and 834 cm −1 due to external asymmetric ( ν as ext), internal asymmetric ( ν as int) and internal symmetric ( ν sym int) stretching lattice vibrations, respectively. The value of 975 cm −1 obtained here for the charge-sensitive ν as int of the ZSM-5, was in good accordance with earlier results gained for bare Cu +, Cu +–CO or Cu 2+ containing ZSM-5. The broadening of the 975 cm −1 band to 1010–990 cm −1 indicated an incomplete ion-exchange of H + by Na + obtained during the preparation of NaH-ZSM-5 from H-ZSM-5. Depending on the amount, water markedly influenced almost the whole FTIR spectra of NaH-ZSM-5, increasing more or less the intensities of the ν as ext, ν as int and ν sym int bands. At high H 2O concentration, the H 2O ligands in the aquacomplex completely screened the positive charge of Na + ions. As a result, the perturbation effect on ν as int disappeared, since ν as int blueshifted to 1010 cm −1. Strongly H-bonded interactions with Fermi resonance and the formation of H +(H 2O) n was highly suggested by the FTIR spectra at medium and high concentration of H 2O, respectively. Even at higher temperatures, a part of silanol OH groups revealed unusually strong interactions with H 2O molecules retained probably by the Na + cations. The 880 cm −1 band attributed to out-of-plane OH bending mode and the ν as int and ν sym int bands varied in relation to each other, in three different stages.