A kinetic study on isophorone synthesis via self-condensation of supercritical acetone catalyzed by aqueous KOH in a tubular flow reactor was conducted. The intention was to improve the isophorone selectivity, with the main approach to reduce the by-products higher-boiling than isophorone. A higher isophorone selectivity was obtained with increased reaction temperature. The structures of main products were determined by gas chromatography-mass spectrometry and nuclear magnetic resonance analysis. The key reaction pathways and a corresponding kinetic model were proposed. The fitted apparent activation energies indicated the reason of isophorone selectivity improvement in that high temperature favors reversion of reversible by-products.