The three-layer ONIOM(B3LYP/6-311 +G**:HF/3-21G*:MNDO) method was used to model the vibrational frequencies of acid sites in zeolite ZSM-5. Bridged OH and OD groups (both types 1 and 2) in eight crystallographically different locations were studied. The calculated anharmonic stretching vibrational frequencies of bridged hydroxyl groups are in good accordance with experimental measurements. The results of calculations point to the heterogeneity of acid sites due to different aluminium sites in zeolite lattice, and due to several probable locations of acid sites around aluminium atoms. (C) 2004 Elsevier B.V. All rights reserved.