Mixed-ligand complexes of nickel(II) with 2,2′-bipyridine (bipy) and 3,4-dimercaptotoluene (DMT) and with 1,10-phenanthroline (phen) and 3,4-dimercaptotoluene were prepared and characterized. These complexes give sharp 1H nuclear magnetic resonance (NMR) spectra in dimethyl- d 6-sulphoxide which suggests that they are diamagnetic and square planar. They show five absorption bands in the region 250 – 600 nm. One of these bands shows a bathochromic shift of about 50 nm when the solvent is changed from CH 3OH to CHCl 3. They also act as physical quenchers of the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by haemotoporphyrin IX with quenching rate constants of 6.8 × 10 9 and 1.1 × 10 10 M −1 s −1 for [Ni(bipy)(DMT)] and [Ni(phen)(DMT)] respectively. These values approach the quenching rate constant (1.1 × 10 10 M −1 s −1 of a diamagnetic square planar complex of bis(di- n-butyldithiocarbamato)nickel-(II) which is a singlet molecular oxygen quencher.