The Beckmann rearrangement has been studied over a range of solid acids, including zeolites and modified aluminas, in the temperature range 250-380 degrees C. Generally ketoximes rearrange to the corresponding lactams, but acetoximes dehydrate to the nitriles. Coke formation was a major problem associated with all solid acids tested, giving rise initially to lowering in conversion and later to lowering in selectivity to the lactam product. There was a direct relationship between levels of coke formation and the decline in catalytic activity. Because coke formation was most prevalent over those catalysts with the largest numbers of surface basic sites, it is proposed that caprolactam polymerization and condensation reactions of byproducts, such as between aniline and cyclohexanone, involve surface basic sites. By contrast, there was a direct relationship between the surface concentration of acidic sites of intermediate strength and the rate of caprolactam formation from cyclohexanone oxime.