Amberlyst-15 has been found to be an efficient catalyst for the synthesis of 1,8-dioxo-octahydroxanthenes and 1,8-dioxo-decahydroacridines in excellent yields. The former have been synthesized from aromatic aldehydes and 5,5-dimethyl-1,3-cyclohexanedione while the latter from this mixture along with amines. The method is an easy access to functionalized xanthene and acridine derivatives. The catalyst can be reused. (c) 2005 Elsevier B.V. All rights reserved.