The equilibrium tautomeric composition in aqueous solution of 6-deoxy-L-psicose and 1-deoxy-D-tagatose was elucidated by means of C-13 NMR spectroscopy at 15-50 degrees C. The molar absorption coefficient of their respective carbonyl group, epsilon(C=O), was determined from the linear relationship between the absorbance around 280 nm and molar concentration of the acyclic carbonyl form. It was found that epsilon(C=O) for 6-deoxy-L-psicose was almost the same as for D-psicose, whereas epsilon(C=O) for 1-deoxy-D-tagatose was reduced by ca. 25% compared to D-tagatose.