In thioanilides, cis-trans isomerism has been clearly differentiated from thioamide – thiolimidic acid tautomerism. The isomer ratio has been related to structural and environmental influences on the thioamide group by means of infrared and proton magnetic resonance spectroscopy. In the thioanilides, the thioamide group does not lie in the ring plane unless constrained to do so by hydrogen bonding between the thioamide proton and a suitable ortho substituent.