Kinetics of the substitution reactions between monofunctional Au(III) complexes, Au(dien)Cl 2+ , Au(bpma)Cl 2+ and Au(terpy)Cl 2+ (dien = 3-azapentane-1,5-diamine, bpma = di-(2-picolyl) amine, terpy = 2,2′;6′,2″-terpyridine), and biologically relevant sulphur ligands, namely glutathione (GSH), l -methionine ( l -Met) and l -cysteine ( l -Cys), were studied in 0.1 M HCl (pH = 1.0). The reactions were followed under pseudo -first-order conditions as a function of ligand concentration and temperature using stopped-flow spectrophotometry. The Au(terpy)Cl 2+ complex proved to be more reactive than the Au(bpma)Cl 2+ and Au(dien)Cl 2+ complexes. The reactivities of the nucleophiles follow the same order for all three complexes, viz. l -Met > GSH >  l -Cys. Values of the activation parameters of the reactions support an associative substitution mechanism. In order to confirm that these monofunctional Au(III) complexes undergo a single substitution process in strongly acidic medium, the reaction between Au(terpy)Cl 2+ and l -Met was studied by HPLC. At pH = 1.0, only one reaction product was detected.