The relative affinity of the cationic triangular metallaprism, [(pCH(3)C(6)H(4)Pr(i))(6)Ru(6)(tpt)(2)(dhbq)(3)](6+) ([1](6+)), for various amino acids, ascorbic acid, and glutathione (GSH) has been studied at 37 °C in aqueous solutions at pD 7, using NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). The metallaprism [1](6+), which is constituted of six (pCH(3)C(6)H(4)Pr(i))Ru corners bridged by three 1,4-benzoquinonato (dhbq) ligands and connected by two 2,4,6tri(pyridin4yl)1,3,5-triazine (tpt) triangular panels, disassembled in the presence of Arg, His, and Lys, while it remains intact with Met. Coordination to the imidazole nitrogen atom in His or to the basic NH/NH(2) groups in Arg and Lys displaces the dhbq and tpt ligands from the (p-cymene)Ru units, and subsequent coordination to the amino and carboxylato groups forms stable N,N,O metallacycles. The binding to amino acids proceeds rapidly, as determined by NMR spectroscopy. Interestingly, solutions of [1](6+) are able to catalyze oxidation of the thiol group of Cys and GSH to give the corresponding disulfides and of ascorbic acid to give the corresponding dehydroascorbic acid. Competition experiments with Arg, Cys, His, and Lys show the simultaneous formation of one single adduct, the (p-cymene)Ru-His complex, and oxidation of Cys to cystine. Furthermore, the (p-cymene)Ru-His complex formed upon the addition of His to [1][CF(3)SO(3)](6) is able to oxidize Cys to cystine much more efficiently than [1](6+). These results provide evidence against interaction with proteins as process in the release of encapsulated guest molecules. Oxidation of Cys and GSH to give the corresponding disulfides may explain the in vitro anticancer activity of [1](6+).