Raman spectra of the C-120 crystalline fullerene dimers, synthesized by high-pressure/high-temperature (HPHT) treatment of the C-60 fullerite, were measured at ambient and elevated temperatures (T > 130 degrees C) in order to study the kinetics of their thermal decomposition. Measurements exhibit an intensity decrease of the dimer-related pentagon pinch (PP) mode and an increase of the monomer's one. The relative intensity of the dimer's PP mode decreases exponentially with the thermal treatment time and becomes faster at elevated temperatures. The activation energy E-A of the dimer decomposition, obtained from the Arrhenius dependence of the exponential decay time constant on temperature, is (1.71 +/- 0.06) eV. (C) 2016 Elsevier B.V. All rights reserved.