The synthesis and properties (NMR, UV absorption, fluorescence emission spectra, and fluorescence quantum yield) of umbelliferone, 4-isopropylumbelliferone, and 4-carboxymethylumbelliferone are described. The optimal conditions for the formation of these umbelliferones from substituted malic acids and resorcinol have been determined. This forms the basis for a rapid and sensitive fluoremetric assay of enzymes catalyzing aldol condensation reactions involving acetyl coenzyme A. © 1969.