The nature and location of the active sites for the Beckmann rearrangement present in microporous and mesoporous solids have been investigated by comparing the results obtained for oximes of different molecular size, using in situ infrared spectroscopy. The rearrangement of acetophenone and cyclododecanone oximes into acetanilide and laurolactam, respectively, was studied using siliceous and Al-containing zeolites Beta and MFI and mesoporous MCM-41 as catalysts. The results indicate that Bronsted acid sites as well as strongly hydrogen bonded silanol groups and silanol nests, located in the pores of zeolites and MCM-41, are active in the reaction. When the external surface or the outer shell of MFI type crystals is considered, the bridging hydroxyl groups appear to be active, whereas no activity in the Beckmann rearrangement of cyclododecanone is observed over silanol groups. (c) 2006 Elsevier Inc. All rights reserved.