Vinyl-substituted silphenylene siloxane elastomers (VSPSEs) with variable vinyl content were synthesized using the disilanol-diaminosilane polycondensation method. High molecular weight elastomers were obtained by using carefully purified monomers. The polymers were characterized by gel permeation chromatography, differential scanning calorimetry (DSC), and thermogravimetric analyses (TGA). Analysis by Si-29 NMR established that the samples have exactly alternating chemical structures. VSPSEs have low glass transition temperatures (T-g＇s) ranging from -26 to -63 degrees C. Substitution of the methyl group on silicon with phenyl increases the T-g as well as the TGA residues in both air and nitrogen. TGA experiments showed that the VSPSEs synthesized in this study have the highest degradation temperatures reported so far. The TGA residues at 900 degrees C increased to 70% in nitrogen and 57% in air as the vinyl content increased. Furthermore, remarkable isothermal weight losses were shown by the VSPSEs. For example, the elastomer with one vinyl group per repeating unit had a weight loss of 0.7% in nitrogen and 3% in air after 5 h at 400 degrees C. Blends of VSPSEs with conventional styrene butadiene rubbers (SBR) and also with styrene-butadiene-styrene triblock copolymers (SBS) were prepared using solution blending. DSC studies indicated that these blends were not miscible. Cross-linking the blends broadened the T(g)s for the blend constituents indicating a small degree of interfacial phase mixing. Thermal-oxidative stability of the blends is intermediate between that of the VSPSEs and the SBR or SBS.