Cycloketonization of dimethyl hexanedioate into cyclopentanone was investigated over pure CeO2 at a temperature between 350 and 475 degrees C. The conversion of the diester increased with raising the reaction temperature. The selectivity to cyclopentanone, however, decreased whenever the conversion increased. The major by-product other than methanol was 2-methylcyclopentanone. It was found that methylation of cyclopentanone with methanol into 2-methylcyclopentanone proceeded over CeO2. The decrease in the selectivity to cyclopentanone at high conversion was mainly caused by a consecutive reaction of cyclopentanone into 2-methylcyclopentanone due to alkylation with methanol. (C) 2006 Elsevier B.V. All rights reserved.