The effect of metal ions on the, reduction of 1,10-phenanthroline-5,6-quinone ( 1), 5,8-quinolinequinone (II) and 6,7-dichloro-5, 8-quinolinequinone (III) has been investigated in 50% dimethylsulfoxide+water solvent. 1 contains the 1,10-phenanthroline structure in both its quinone and hydroquinone forms, while II and III contain the 8-hydroxyquinoline structure in the hydroquinone forms. Complexation of the hydroquinones of II and III by metal ions causes positive shifts in the quinone half-wave potentials. These shifts have been used to calculate conditional formation constants for Pb 2+( II) and Pb 2+ and Zn 2+( III). The quinone form of I binds strongly to Ni 2+, Co 2+ and Zn 2+ but not to Ca 2+. Mg 2+. Mg 2+. Mn 2+ and Pb 2+. With the latter four metals, binding to the hydroquinone from of I was detected and formation constants were determined. In addition to binding both the quinone and hydroquinone forms at the nitrogen atoms, Ni 2+ Co 2+ and Zn 2+ formed complexes at the 1,2-dihydroxy site of the hydroquinone of I.