The interaction of acetic anhydride with the Na- and H-forms of zeolite BETA, as well as with materials where copper or iron has been introduced into BEA by ion exchange, has been studied by temperature programmed desorption (TPD). In all cases acetic acid is a main desorption product in the temperature range 370-570 K. Ketene desorption is also significant, although it is argued that much of it is retained in the zeolite pores, where it undergoes further decomposition into carbon monoxide, hydrogen and water. There is an inverse correlation between the quantity of ketene desorbed and the activity of the catalysts in the catalytic acylation of anisole by acetic anhydride to yield p-methoxyacetophenone. The significance of this observation is discussed and an acylation mechanism involving ketene is proposed. (C) 2003 Elsevier B.V. All rights reserved.