The solid base catalysed aldol condensation of acetone was performed over a CsOH/SiO2 catalyst using a H-2 carrier gas. The products observed were diacetone alcohol, mesityl oxide, phorone, iso-phorone and the hydrogenated product of mesityl oxide, methyl isobutyl ketone. Deuterium tracer experiments were performed to gain an insight into the reaction mechanism. Over 10% CsOH/SiO2 deuterium exchange of acetone was very facile with conversion up to 92%. Isotope studies have revealed that the hydrogenation step from mesityl oxide to MIBK occurs though the addition of protium and not via deuterium. This is a result of the dilution of the deuterium pool on the catalyst surface due to facile HID exchange of acetone. The formation of isophorone is proposed to occur via an aldol condensation of 4,4'-dimethyl hepta-2,6-dione and not through the internal 1,6-Michael addition of phorone. A mechanism is proposed. (c) 2005 Elsevier B.V. All rights reserved.