The rearrangement of cyclohexanone oxime to epsilon-caprolactam was investigated over heterogeneous catalysts, especially over highly siliceous zeolites with a MFI structure. The aim of this work was to determine the active sites on these catalysts, rather than to optimize their catalytic performance. Amorphous silica and the silicalite-1 zeolite were chemically modified with aqueous acidic or basic solutions. Silicalite-1 was also treated hydrothermally at temperatures above 400 degrees C. Catalytic reactions were carried out with the initial materials and their modifications in a fixed-bed reactor. Characterization of the catalyst with FT-IR, nitrogen sorption, and XRD revealed significant differences and gave important information about the active sites in the reaction. Silanol nests on the external surface of the highly siliceous ZSM-5 zeolite were most suitable for the Beckmann rearrangement.