Aldol condensation of cycloketone(s) is an efficient reaction path to synthesize polycyclic compounds via C-C bond formation. In this work, aldol condensation of cyclopentanone and/or cyclohexanone over monoclinic ZrO2 has been investigated by combination of experiments and DFT calculations. Both self condensation and cross-condensation of cyclopentanone and/or cyclohexanone were carried out at 130 degrees C in a solvent-free condition under atmospheric pressure. Reactant selectivity and product distribution were analyzed. DFT calculations were performed to investigate the mechanism and reactant selectivity of cycloketone condensation on ZrO2, showing the favored cross-condensation between cyclopentanone and cyclohexanone is ascribed to the formation of a metastable state of a seven membered ring that lowers activation barrier during C-C bond formation. (C) 2018 Elsevier Inc. All rights reserved.