The complex formation equilibria of nickel(II) with 1,4,7,10-tetraazacyclododecane (cyclam, L) were investigated in 0.1 mol L-1 ionic strength medium, by KCl at three different temperatures (25 degrees, 35 degrees and 45 degrees C). In this research, potentiometric and spectroscopic titrations were applied that were not performed before in cited studies. The protonation constants of H2L2+ and HL type ions of cyclen were defined; they were in good agreement with other works. The formation constant of NiH2L4+ type complex was determined and thus its occurrence in lower pH range was proved that had not taken place in other Ni(II)-cyclam studies. In higher pH range the existence of NiL2+ type complex was identified and its formation constant was defined. It was noticed that the stabilities of either NiH2L4+ and NiL2+ complexes decrease with temperature. By means of speciation diagram the existences of NiH2L4+ and NiL2+ complexes were also verified. The overall changes in thermodynamic parameters (Delta H, Delta S and Delta G) associated with NiL2+ complex formation equilibrium were evaluated graphically.