The protonation and stability constants of the binary Cu 2 complexes with 1,1'-[(1H-pyrazole-3,5-diyl)bis(methylene)]bis(octahydro-1H-1,4-triazonine) were determined by potentiometric titrations at 25 o and I=0.5 M (KNO 3 ). The results show that a series of mononuclear [Cu(LHn)] (n=1, 0, -1, -2) and dinuclear species [Cu 2 (LHm)] (m=0, -1, -2, -3) are formed. Structures are proposed for them based on their absorption spectra and results previously obtained from X-ray diffraction studies. Complex [Cu 2 (LH - 1 )] 3 , in which the pyrazole bridging group is deprotonated, is especially interesting since it is able to bind exogenous ligands Z such as N 3 - , pyrazole and 1H,1'H-4,4'-methanediyl-bis-pyrazole (bispyr) to give ternary species [Cu 2 (LH - 1 )Z] and [[Cu 2 (LH - 1 )] 2 Z], the stabilities of which have been measured by spectrophotometric titrations. In the case of bispyr the existence of the tetranuclear complex [[Cu 2 (LH - 1 )] 2 (bispyr)] 2 has been proven by ES-MS.