Abstract N-Cl, N-Br and N-J secondary amides have been investigated in the i.r. From the data on N-H and CO stretching vibrations in carbon tetrachloride solution it appears that the N-halogen aliphatic amides show cis-trans isomerism across the C-N amide bond, the trans form being predominant. In the N-halogen benzamides only one isomer is present. In solvents that are more basic than carbon tetrachloride the cis isomer is lost completely. The amide II band at ~ 1550 cm −1, which is considered to be highly characteristic trans secondary amides is absent in the spectra of the trans compounds in solution. Solid N-Cl and N-Br amides do not show the amide II band but N-I amides show a strong absorption at ~ 1550 cm −1. The characteristic absorption bands of solid N-halogen secondary amides are given in the region 4000–4400 cm −1. From the i.r. data no conclusion can be drawn whether solid N-halogen secondary amides occur in a cis or trans configuration.