The far-infrared dielectric properties of sugars in the condensed state are dominated by vibrational modes of their intermolecular hydrogen bonded network. We have used terahertz time-domain spectroscopy to measure the full dielectric function of the monosaccharides glucose and fructose and the disaccharide sucrose in the frequency range 0.5-4.0 THz (16-130 cm(-1)). In polycrystalline sugars we observe a series of distinct absorption lines originating from the lowest intermolecular vibrational modes, whereas in amorphous sugars a broad, featureless absorption spectrum is observed. Our measurement of an anomalous temperature dependence of the absorption line positions in both protonated and deuterated sucrose shows that the effective potential of the weakest intermolecular vibrational modes in sucrose is determined by a balance between the hydrogen bond strength and van der Waals forces. (C) 2003 Elsevier Science B.V. All rights reserved.