Low-frequency torsional vibration modes of the conjugated polyene chain of the biologically important chromophore retinal are of major relevance when studying its photoisomerization dynamics. We present absorptivity and dispersion spectra of three different retinal isomers in the far-infrared region between 10 and 100 cm(-1) (0.3-3.0 THz) measured by terahertz time-domain spectroscopy at 298 and 10 K. At low temperatures the observed broad absorption bands resolve into narrow peaks correlating to torsional modes of the molecule. Comparison of the absorption spectra of different retinal isomers enables the approximate localization of the modes within the molecule. Experimental linewidths and transition strengths are reported. (C) 2000 Elsevier Science B.V. All rights reserved.