[B]-ZSM-5 is a very active and selective catalyst for the vapor-phase Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam with high selectivities of up to 95% at 300 degrees C and 100 mbar at complete initial conversion. The nature of the catalytically active sites on [B]-ZSM-5 was determined by investigating the influence of postsynthesis modifications, e.g., calcination under air, acid/base treatment, and hydrothermal treatment on the selectivity. Using FT-IR various hydroxyl groups and silanol nests on the surface of the [B]-ZSM-5 zeolites were found to be the active sites for the Beckmann rearrangement. Further investigations concerned the negative effect of adsorbed water on [B]-ZSM-5, which led to a decrease of its catalytic activity. This effect was shown to be reversible and could be explained with the hydration of boron oxide species on the surface. Experiments with as-synthesized and already calcined [B]-ZSM-5 showed the influence of the boron oxide species, which enhanced the conversion compared to the material without extraframework boron from 70% to 98%.