The stoichiometry of the thermal decomposition and the stereochemistry of the following compounds was studied : NiR 4 Cl 2 (I), NiR 4 Br 2 (II), NiR 4 I 2 .2H 2 O (III) and NiR 4 (NCS) 2 (IV) (R=3-pyridylcarbinol, ronicol). In complexes I and II the loss of the volatile ligands (on the TG curves) occurs in three steps (-2R, -R, -R), in complex III in four steps (-2H 2 O, -2R, -R, -R) and in complex (IV) in one step (-4R). According to the quasi-equilibrium decomposition temperatures, the thermodynamic stability of NiR 4 X 2 complexes can be ordered in the sequence (according to X) :Cl