Hydrogen–deuterium exchange in tricarbonylchromium complexes of anisole, ethylbenzoate, andN,N-dimethylaniline has been studied for solutions in ethanol-O-d1, containing sodium ethoxide. The extent of deuteration has been estimated mass spectroscopically and the position of deuteration by n.m.r. spectroscopy. In this kinetically controlled reaction ortho-substitution has been found to be predominantly preferred in case of anisole complex, whereas random substitution was observed (n.m.r.) for ethylbenzoate complex. The results are interpreted in terms of chelative, inductive, mesomeric, and steric effects of the substituents.