Studies on the chelate complexes of copper(II) with the bidentate ligands, 1,4-diphenyl-2,3-dimethyl-1,4-diazabutadiene (PMB) and 1,4-di(p-methoxyphenyl)-2,3-dimethyl-1,4-diazabutadiene (MPMB), have been carried out. On the basis of the elemental analysis and molar conductivity, the complexes have been characterized as [Cu(PMB)Cl2], [Cu(PMB)2](ClO4)2, [Cu(MPMB)Cl2] and [Cu(MPMB)2](ClO4)2. Both diazabutadienes are bidentate via nitrogen atoms. The temperature dependence of the magnetic susceptibility and the electron paramagnetic resonance, i.r. and u.v.-vis spectra are reported. All compounds appear to be monomers.