Ab initio molecular orbital calculations using Hartree-Fock theory and Moller-Plesset perturbation theory have been used to study the interaction of H2O with the Bronsted acid site in the zeolite H-ZSM-5. Aluminosilicate clusters with up to 28 T atoms (T = Si, Al) were used as models for the zeolite framework. Full optimization of a 3 T atom cluster at the MP2/6-31G(d) level indicates that the "ion-pair" structure, Z(-)... HOH2+, formed by proton transfer from the acid site of the zeolite (ZH) to the adsorbed H2O molecule, is a transition state, while the "neutral" adsorption structure, ZH ... OH2, is a local energy minimum. Partial optimization of a larger 8 T cluster at the HF/6-31G(d) level also gave results suggesting that the ion-pair structure is a transition state. Calculations were carried out to obtain corrections for high levels of theory, zero-point energies, and larger cluster size. The resulting energy difference between the neutral and ion-pair structure is small (less than 5 kcal/mol and possibly close to zero). The interaction energy of ZH ... OH2 is 13-14 kcal/mol, in agreement with experiment. We find that addition of a second H2O molecule to Z(-)... HOH2+ in the 3 T atom cluster stabilizes the ion-pair structure, Z(-)... H(OH2)(2)(+), making it a local energy minimum. Finally, calculated vibrational frequencies for a 3 T atom cluster are used to help interpret experimental IR absorption spectra.