The local density functional theory coupled to the molecular cluster approach has been used to study the chemisorption of Bronsted acids (H2O, H2S, HCN, CH3OH and CH3SH) on the ZnO(0001) polar surface. Geometrical parameters and vibrational frequencies for selected species molecularly and dissociatively chemisorbed have been computed. The agreement with literature experimental data, when available, has been found to be good. The nature of the interaction between the conjugate base of the examined Bronsted acids and the Lewis acid site available on the surface has been elucidated, confirming its leading role in determining the actual relative acidity scale obtained by titration displacement reactions. The strength of this interaction follows the order OH(-)approximate to CN- > CH3O- > SH- > CH3S-
