Schiff base ligands HL1-HL6 have been prepared from the reaction of 2,6-diformyl-4-t-butylphenol and 2,6- diformyl-4- methylphenol with various aromatic amines in ethanolic solution. The Schiff base ligands 2,2'-dipyridine (dp) mixed- ligand Cu-II complexes have been obtained. Mixed-ligand Cu-II complexes containing the dp ligand have ionic nature and they conduct the electricity in solution media. The complexes have been obtained in two different forms: one of them is [Cu-2(L-n)Cl-3] (n: 1, 2, 3, 4, 5 and 6) and other complexes have the general formula [Cu-2(L-n)(dp)(2)]3Cl. Ligands and their complexes have been characterized by elemental analyses, FT-IR, electronic spectra, molar conductance, H-1(C-13)- n.m.r. and mass spectral data. Their stoichiometric protonation constants have been determined potentiometrically in dioxan using a combined pH electrode at 25 degrees C, under a nitrogen atmosphere. For the calculation of the protonation constants, PKAS computer programme has been used. The effects of the substituents on the protonation constants and the additivities of these effects are discussed. The antimicrobial activity studies of the ligands and their complexes have been studied against the Bacillus megaterium, Micrococcus luteus, Corynebavterium xenosis, Enterococcuc faecalis, bacteria and Saccoramyces cerevisia, yeast. The catalytic properties of the complexes have been studied on the ascorbic acid, catechol and 2,6-di-t-butylphenol substrates. Thermal behaviour of the complexes has been studied by thermal techniques.