The thermogravimetric analysis resulted to be an analytical technique, very illustrative that allowed us to determine the thermal stability, molecular structure and the formation process of cross-linked structures, in the synthesis of poly(methylsiloxane). Poly(methylsiloxane) was synthesized by cationic ring-opening polymerization of the cyclic monomer 1, 3, 5, 7-tetramethylcyclotetrasiloxane. A series of polymers were obtained by varying polymerization times. The initial and maximum decomposition temperatures increase to higher polymerization time until constant values are reached. At short polymerization time mainly linear polymers are formed with small fraction of cross-linked structures. At higher polymerization time partial and total cross-linked polymers are obtained. Formation of cross-linked structures stabilizes thermically the whole polymeric matrix, being the stability proportional to the cross-linking degree. The thermogravimetric analysis allowed us to formulate a method to determine the cross-linking degree of these polymers. FT-IR analysis showed the existence of the absorption band of variable intensity, corresponding to the vibration of the Si-H bond, in all the studied polymers. FT-IR spectra of the residual ceramic powder obtained by pyrolysis showed the typical absorption bands of these materials. The characterization of the polymers was also carried out by thermomechanical analysis. Kinetic decomposition parameters such us pre-exponential factors, decomposition reaction orders and activation energies were determined by thermogravimetry.