Thermogravimetric TG/DTG and calorimetric DSC studies of four types of complexes : (A) Ni(RL)(2)I-2 . 2S, (B) Ni(RL)(2)I-2 . S, (C) Ni(RL)(2)I-2, and (D) Ni(RL)(2)(I-3)(2) were performed. The decomposition steps of these compounds were observed. The important role of the solvent molecules S in stabilization of the crystal structure of the examined complexes is suggested.