The synthesis and characterization of a series of mixed-ligand oxorhenium(V) complexes containing the o-diphenylphosphinophenolato ligand (HL) and model peptide fragments acting as the tridentate coligand are reported. Thus, by reacting equimolar amounts of tiopronin, Gly-Gly, Gly-L-Phe, or glutathione (GSH) peptides on the [(n-C4H9)(4)N] [ReOCl3(L)] precursor in refluxing MeCN/MeOH or aqueous:MeCN/MeOH mixtures, the following complexes were obtained: ReO{[SC(CH3)CONCH2COO][L]}[(n-C4H9)(4)N], 1, ReO{[H2NCH2CONCH2-COO] [L]}, 2, ReO{[H2NCH2CONCH(CH2C6H5) COO][L]}, 3, and ReO{[SCH2CH(NHCOCH2CH2CHNH2-COOH)CONCH2COO] [L]}Na, 4. The compounds are closed-shell 18-electron oxorhenium species adopting a distorted octahedral geometry, as demonstrated by classical spectroscopical methods including multinuclear NMR, X-ray diffraction analyses for 1 and 2 are also reported. By comparative stability studies of complexes 1-3 against excess GSH it was shown that complex 3 containing the bulky C6H5CH2 substituent adjacent to the coordinated carboxylate group of Phe is the most stable complex.