The interaction of [Au(en)Cl2]Cl with L-methionine (Met) and DL-seleno-methionine (Se-Met) was carried out in D2O solution at ∼pH 3.8. 13C NMR spectra were interpreted by comparison with the spectra recorded after the reaction of Met or Se-Met with HAuCl4 and also after oxidation by H2O2. Reactions of [Au(en)Cl2]Cl with Met and Se-Met were found to proceed through the exchange of chloride ligands from complex with Met or Se-Met. Gold(III) reduction takes place during the reaction and en ligand was released from [Au(en)Cl2]Cl complex.