The reaction of a lanthanide(III) nitrate (Ln = Pr, Nd, Gd, Dy, Er) with 3-methoxy-salicylaldehyde(3-OCH(3)-saloH), afforded neutral complexes of the general formula [Ln(3-OCH(3)-salo)(3)], which were characterized by means of elemental analysis, FT-IR spectra, TG-DTA curves, and magnetic measurements. The released products, due to the thermal decomposition were analyzed by on-line coupling MS spectrometer to the thermobalance in argon, allowed to prove the proposed decomposition stages. In order to confirm the stability scale provided on the basis of the onset decomposition temperature, a kinetic analysis of the three decomposition stages was made using the Kissinger equation, while the complex nature of the decomposition kinetics was revealed by the isoconvertional Ozawa-Flynn-Wall method.