Stability constants of some ternary complexes of o-hydroxy aldehydes (HL) with Cu(II) as central metal ion and bis(2-pyridyl)amine (dpa) and 5-nitro-1,10-phenanthroline (5-N-phen) as primary ligands (A) have been determined potentiometrically in 50% aqueous dioxane medium using a computer technique. The values of [Formula: see text] have been compared within the series as well as with the corresponding bipyridyl complexes. The stability order has been explained on the basis of the electron repulsion concept. The complexes have been isolated in the solid state and characterized by elemental analysis, conductance and magnetic measurements, and ir and electronic spectral studies. Shifts in ligand field absorption wave numbers of the mixed ligand complex from the average value of the ligand field absorption wave numbers of [MA2]2+ and [ML2] have been correlated with the potentiometrically determined Δ log K values.