The i.r. spectra of forty aromatic acid chlorides, including mono- and poly-substituted benzoyl chlorides; benzoyl halides and naphthyl, phthalyl and heterocyclic acid chlorides are recorded. The stretching frequencies ( v) and integrated intensities ( A) of the bands in the regions around 1770, and 870 cm −1 are tabulated. No simple relationship was obtained between each of the ( v) and ( A) of these bands and the substituent constants. The latter bands are assigned as v CO and v C-Cl vibrations respectively. The v CO vibrations varies from single to complex absorptions depending on the nature and position of the substituent in benzoyl chlorides, type of halo-atom in benzoyl halides and type of hetero-atom in heterocyclic acid chlorides. The complex carbonyl absorption is discussed in terms of Fermi-resonance effect which is verified on the basis of solvent effects.