We have studied an activation mechanism of cyclohexanone oxime in a cavity of high silica MFI zeolite by using PIO analysis proposed by Fujimoto et al. DFT calculation reveals that the bond length of N-OH becomes longer when water coordinates on oxygen of oxime. The PIO clearly shows out-of-phase interaction between N and O. This out-of-phase interaction is also observed in the PIO of oxime/MFI zeolite cluster model and weakens the N-O bond. Hydrogen bond of Si-OH of nest silanols to oxime is a trigger of vapour phase Beckmann rearrangement. (C) 2003 Elsevier B.V. All rights reserved.