This study presents the synthesis of the poly(succinimide)-co-poly(ethylene glycol) block copolymer (PSI-co-PEG) as a matrix for ferrite encapsulation via an in situ polycondensation reaction, by using manganese acetate as a catalyst and dodecane as a solvent. The copolymer and magnetic composite with ferrite in its composition were characterized by FT-IR spectroscopy and thermal analyses (TA) coupled with FT-IR and mass spectrometry (MS). The behaviors while heating and at thermal stability for the copolymer and magnetic composite are different. As a function of the heating rate, the block copolymer presents 3-4 thermal degradation stages, while the magnetic composite decomposed in a maximum of two stages; as compared with the magnetic composite, the weight losses are smaller in the case of the copolymer matrix. The ferrite in the magnetic composite may have an autocatalytic effect that contributes to the acceleration of the thermal degradation process. The analysis of the mass and FT-IR spectra for the released gases in the thermal decomposition process does not reveal an unusual difference, but the temperatures for their identification are different. Generally, using TG/FTIR/MS simultaneous analysis, more gaseous species were determined from the copolymer and magnetic composite in the temperature range of 30-600 degrees C.