Recently, highly luminescent lanthanide complexes with mixed ligands such as and β-diketone and two herobiaryl ligands have been synthesized and suggested as promising light-conversion molecular devices as described by several authors. The synthesis, luminescence, quantum yields, spectroscopic characteristics, structure, among others properties of these complexes, including the possibility of production thin film films from these materials have been discussed, but little is known about their thermal decomposition kinetics. In this work we report the determination of kinetic parameters of thermal decomposition of the complexes Gd(thd)3phen and Er(thd)3phen (thd=2,2,6,6-tetramethyl-3,5- heptanodione; phen=1,10-phenantroline) using the method proposed by Ozawa (1965). The kinetic parameters obtained for activation energy were: 90 and 87 kJ.mol-1, the values of frequency factor were: 2.3x107 and 2.0x107 s-1 for Gd(thd)3phen and Er(thd)3phen, respectively. On the kinetic energy of the complexes, we notice the following order in thermal stability: Gd(thd)3phen < Er(thd)3phen.