Polymethylvinylsiloxane (PMVS) were prepared by pre-hydrolysis/condensation of methylvinydichlorosilane (MVDCS) and dimethyldichlorosilane (DMDCS) followed by catalysis equilibrium copolymerization by sulphuric acid (H₂SO₄) and all manipulations in the experiments were performed under ambient condition. The PMVS were characterized by Fourier transform infrared spectroscopy (FTIR). This method has provided information about the structure of the polymer. Thermal properties of the polymethylvinylsiloxane were studied by using thermogravimetric analysis (TGA) indicate that the polymethylvinylsiloxane showed high thermal decomposition temperatures and low residue yields as a result of oxidation or degradation of methyl/vinyl groups.