To complete preceding spectroscopic investigations of the gaseous acetonitrile and chloroacetone adsorbed on solid aluminum chloride, we have examined the spectra of gaseous chloroacetonitrile adsorbed on the same catalyst. The behaviour of absorption bands CN and CC of the adsorbed molecule is analogous with the behaviour of acetonitrile; on the contrary the absorption band CCl has a different behaviour to that of chloroacetone. We suggest an explanation of the phenomenon according to the different stereochemistry of the adsorbed molecules.