Abstract The infrared spectrum between 33 and 400 cm −1 has been examined in the vapor and liquid phases for the following 26 compounds: o-, m-, and p-F, Cl, Br, and CH 3 benzaldehydes; pyridine-2-, -3-, and 4-aldehydes; acetophenone and its o-, m-, and p- F derivatives; furan-2-aldehyde; and several monofluorostyrenes and -nitrobenzenes. Torsional frequencies were sought, and have been assigned for all but the last two groups of compounds. For all the meta-substituted benzaldehydes, for a few of the ortho ones, and for furan-2-aldehyde evidence has been found for the presence of two rotational isomers in the vapor. The parameters V 1 and V 2 of an approximate potential function for the internal rotation have been evaluated. In the meta benzaldehydes the O-cis rotamer is the more stable one, whereas in the ortho compounds and in furan-2-aldehyde it is the O-trans one. For meta-fluorobenzaldehyde a temperature-dependence study of some bands in the mid-infrared has confirmed the existence of two rotamers, and has given 0.5 kcal/mole as an approximate value for their energy difference in CDCl 3 solution.