Two sucrose hydrates were found by factor analysis (FA) of the mid-infrared attenuated total reflectance (MIR-ATR) spectra of a series of aqueous sucrose solutions: sucrose penta- and dihydrate. The spectra of the isolated hydrates and their abundances as a function of concentration were obtained. Apart from the intensity of the water bands, the two hydrates differ only in a few sucrose bands in the C-O region. From the distribution of the species, the equilibrium between penta- and dihydrate was deduced: 3H(2)O + C12H22O11. 2H(2)O reversible arrow C12H22O11. 5H(2)O. The pentahydrate equilibrium constant was found to be K-e = (4.1 +/- 0.3) x 10(-5) L-3 mol(-3). The two hydrates observed in aqueous solution are not found in the solid state. Analysis of the integrated intensity of the OH stretching band of sucrose-H2O mixtures shows that the OH vibrations in both the sucrose and the associated water are modified in the mixtures, indicating strong intermolecular interactions that influence the band intensities.