The protonation and stability constants of the binary Cu2 complexes with 1,1'-[(1H-pyrazole-3,5-diyl)bis(methylene)]bis(octahydro-1H-1,4-triazonine) were determined by potentiometric titrations at 25° and I=0.5 M (KNO3). The results show that a series of mononuclear [Cu(LHn)] (n=1, 0, −1, −2) and dinuclear species [Cu2(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 [Cu2(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 N3−, pyrazole and 1H,1'H-4,4'-methanediyl-bis-pyrazole (bispyr) to give ternary species [Cu2(LH−1)Z] and [[Cu2(LH−1)]2Z], the stabilities of which have been measured by spectrophotometric titrations. In the case of bispyr the existence of the tetranuclear complex [[Cu2(LH−1)]2(bispyr)]2 has been proven by ES–MS.