Abstract The reactions between dithizone (H 2 Dz, ( 1 )) or potassium dithizonate (K + HDz − , ( 2 )) and phenylmercury(II) chloride gives PhHg(HDz), ( 3 ). Complex ( 3 ) is photochromic. In dichloromethane, the blue photo-exited state of ( 3 ) exhibits a first order relaxation process to regenerate the orange ground state with rate constant 0.00053 s −1 . The half life of this relaxation is ca. 1300 s. Electrochemically, on cyclic voltammetry time scale, the oxidations of ( 1 ) and ( 3 ) are different. A comparative voltammetric and spectro-electrochemical study of ( 1 ) and ( 3 ) in CH 2 Cl 2 containing 0.1 mol dm −3 [N( n Bu) 4 ][B(C 6 F 5 ) 4 ] revealed that the mercapto group of ( 1 ) can be oxidised in two one-electron transfer steps. A disulphide is first produced and then in a second oxidation step, HDz + is formed. In contrast, complex ( 3 ) shows only one ligand-based oxidation step. Upon complexation with phenylmercury the free mercaptan group of ( 1 ) becomes a stable “metal thioether”, Hg–S–C, which effectively prevents disulphide formation in ( 3 ) upon electrochemical oxidation. Both ( 1 ) and ( 3 ) shows two reduction steps. The electrochemical fingerprint of blue photo-excited ( 3 ) is identical to that of the orange ground state as no new functional groups are introduced upon irradiation; only bond rotation occurs. The different electronic spectra for each of the redox states of ( 3 ), obtained from spectro-electrochemical measurements, revealed that only the ( 3 )/( 3 − ) couple exhibits electrochromic properties.