The synthesis of a series of vinyl-substituted silphenylene siloxane polymers of high molecular weight is described. Si-29 NMR spectroscopy indicates that the polymers possess an exactly alternating microstructure of the silphenylene and siloxane moieties. The materials are completely amorphous with glass transition temperatures between -60 and -86 degrees C. All polymers degrade thermally only above 500 degrees C under both oxidative or inert atmosphere. The char yields at 900 degrees C increase with increasing vinyl content from 30% up to almost 70%. Cross-linking of a small fraction of vinyl groups by hydrosilylation gives materials with elastomeric properties. Dynamic mechanical experiments indicate that the bulk of vinyl groups cross-links thermally at temperatures above 230 degrees C. Therefore, the temperature window for possible applications of these materials as fire-safe elastomers extends from about -86 degrees C to approximately 230 degrees C.