In the mid-infrared attenuated total reflectance (MIR-ATR) spectra of aqueous D-glucose and D-fructose solutions, two hydrates were found by factor analysis ( FA) for each sugar, D-glucose penta- and dihydrates and D-fructose penta- and monohydrates. We obtained the spectra and abundances for these hydrates as a function of carbohydrate concentrations. The biggest difference in these spectra lies in the CO stretch region. From the distribution of the species, the equilibrium between D-glucose pentahydrate and dihydrate is 3(H2O)(2) + 2(C(6)H(12)O(6)(.)2H(2)O) reversible arrow 2(C(6)H(12)O(6)(.)5H(2)O), with the equilibrium constant K-G = (3.2 +/- 0.6) x 10(-5) L-3 mol(-3). For D-fructose, the equilibrium is between pentahydrate and monohydrate, 2(H2O)(2) + (C6H12O6H2O)-H-. reversible arrow C6H12O6. 5H(2)O, with the equilibrium constant K-F (7.1 +/- 1.2) x 10(-3) L-2 mol(-2). The four hydrates are present only in aqueous solutions and cannot be obtained in the solid state.