A new water soluble Re(I) complex with a zwitterionic structure, Bu 4 N[(bpy)Re(CO) 3 (dcbpy)] (where Bu = butyl; bpy = 4,4′-bipyridine; dcbpy = 2,2′-bipyridine-5,5′-dicarboxylate), was successfully synthesized and characterized by elemental analysis, 1 H NMR, FTIR and ESI. Protonation studies in aqueous solutions of the Re(I) complex showed three acid–base equilibriums with pK a1 = 5.0, pK a2 = 3.0 and pK a3 = 2.0. pK a1 was assigned to the protonation equilibrium at bpy while pK a2 and pK a3 could be ascribed to protonation/deprotonation of the two carboxylates groups in the dcbpy ligand. With the aid of TD-DFT calculations the nature of the electronic transitions responsible for the pH-dependent UV–vis spectroscopy of the Re(I) complex was identified. At pH = 7 the lower energy band of the complex has MLLCT Re(CO)3→dcbpy character while at pH < 2 it switches to MLLCT Re(CO)3→bpy . This change in the nature of the lower energy band is responsible for the overall spectral changes in the 350–500 nm range after protonation of the Re(I) complex.