The kinetics of the thermal decomposition of solid complexes of the type Ni(N CS)2Lz (L = pyridine, fl-picoline and quinoline), of pseudooctahedral configuration, were studied by using isothermal methods, on the basis of losses of weight, in the temperature range 90--191 ~ . The most suitable reaction order for all the complexes under investigation was found to be n = 2/3, i.e. the total decomposition rate is determined by the chemical process proper. The calculated values of E~ (in kcal-mole -1) decrease in the following order: Ni(NCS)2py2 (29.4) > Ni(NCS)2(fi-pic)z (27.6) > Ni(NCS)2Q,, (24.3). With increasing volume of the ligand L the reaction rate also increases, and this suggests that the reaction proceeds by dissociative activation. For all the investigated complexes it was found that AH > EA; this may be explained by a several-step mechanism and the complex Ni(NCS).~L is then considered an intermediate.