N-15 solid-state NMR and theoretical calculations were combined to investigate the Beckmann rearrangement of N-15-cyclohexanone oxime and N-15-cycloclodecanone oxime over two MFI-type zeolites as catalysts, H-ZSM-5 and silicalite, containing Bronsted acid sites and silanol groups, respectively. The results demonstrated that the Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam occurs in the interior of the pores of MFI zeolites and that O-protonated epsilon-caprolactam is formed over the Bronsted acid centers. The acid groups placed at the outer shell of the crystals, probably at the pore mouths, are also active; meanwhile, external silanol groups are significantly less active and less selective than the internal ones. (C) 2007 Elsevier Inc. All rights reserved.