The adsorption of water on H-ZSM-5 at room temperature has been studied by means of transmission Fourier transform infrared spectroscopy using O-18-substituted water. From the isotope shifts, the bands at similar to 2800 and similar to 2400 cm(-1) have been conclusively assigned to the so-called A and B bands of the Z-OH water hydrogen-bonded complex on the zeolite. The shifts of other bands were also found to conform to the feature expected for the H-bonded model.