The absorption spectra connected with the torsional vibrations of long-chain substances are calculated. Ketones are treated in detail, but the results can also be applied to other long-chain molecules with simple dipolar groups. It is shown that the torsional frequencies of a long-chain ketone are very similar to those of the parent hydrocarbon. But the presence of the ketone group makes the vibrations active in the spectrum. For a molecule with L carbon atoms there are L-3 torsional vibrations. For isolated molecules in the plane configuration, the frequencies range from a maximum $\omega_m$ in the far infra-red, down to low values. For an asymmetric ketone all the torsional frequencies are active, while for a symmetric ketone only half of them are active. However, the absorption intensity for frequencies near $\omega_m$ is expected to be very weak. In the solid state, the high and medium torsional frequencies are hardly affected by intermolecular interaction. In this region, therefore, the frequencies calculated for isolated molecules should be approximately correct. The low torsional frequencies, on the other hand, are strongly affected by intermolecular forces in the solid. This effect is discussed, and the absorption due to rigid libration of the molecules is also considered.