Cs exchanged phosphotungstic acid is a highly efficient and environmentally benign solid acid catalyst for the liquid-phase Beckmann rearrangement of ketoximes to the corresponding amides. The catalysts CsxH3-xPW12O40 (x = 1.5, 2, 2.5 and 3) were prepared by a titration method. The characterization results indicated that the primary Keggin structure remained intact after exchanging the protons with Cs ions. Moreover, the Cs exchanged catalysts were insoluble and exhibited larger BET surface area than the parent acid. The catalysts exhibited high reactivity and selectivity for the formation of epsilon-caprolactam, the precursor of Nylon 6, from cyclohexanone oxime. The catalyst can be recovered after reaction without any structural transformation. (C) 2009 Elsevier B.V. All rights reserved.