A high-resolution continuous-wave terahertz spectrometer has been used to investigate the absorption spectrum of biotin in a polyethylene matrix from 6 to 115 cm(-1). Gaussian linewidths from least squares fits at 4.2 K are found to increase from 1 to 5 cm(-1) over this region establishing conservative lower limits on the vibrational lifetimes from 5 to 1 ps. The calculated partition function of 1.0 at 4.2 K is shown to increase to >10(8) at 298 K indicating enormous changes in the sequence level populations with increasing temperature. Lineshape models that include mechanical anharmonicity were therefore necessary to obtain satisfactory fits at 298 K. Anharmonicity factors (chi(e)nu(e)/nu(e)) range from 0.1% to 1% and establish their importance for refining model predictions. Published by Elsevier B.V.