We have measured the absorption spectra of alpha-hydroquinone and methane- and methanol-loaded beta-hydroquinone clathrates up to 3 THz frequency range using terahertz time-domain spectroscopy (THz-TDS). The alpha-hydroquinone exhibits 11 resonances whereas methane- and methanol-loaded beta-hydroquinone clathrates show only 1 resonance. The observed THz resonances represent the intermolecular and lattice vibrations of the hydroquinone frameworks, but seem to be insensitive to the encaged guest species. The indices of refraction are found to be ca. 1.6 for alpha-hydroquinone and methane-loaded beta-hydroquinone clathrate and 1.7 for methanol-loaded beta- hydroquinone clathrate, and the anomalous dispersions are well-defined at the resonance frequencies. (C) 2008 Elsevier B. V. All rights reserved.