The molar absorption coefficient of carbonyl groups, epsilon(C=O), of acyclic D-fructose, D-psicose, D-tagatose, and D-sorbose was estimated from the linear relationship between the absorbance at 280 nm and the carbonyl group concentration estimated via C-13 NMR at various temperatures. The obtained epsilon(C=O) values for ketohexoses were 3-4 times greater than for acetone and dihydroxyacetone. De localization of the carbonyl pi electrons is thought to be the cause of the enhanced absorbance in acyclic ketohexose molecules.