Selected members of the SiB polymer family were developed as elastomers for high temperature applications. To this end, polymerization procedures, processing technology, and testing techniques were extensively established. Progress of this work was judged from basic measurements, such as tensile strength, elongation, and hardness which were regularly performed at temperatures ranging from —80° F to 800° F, altogether on more than 300 molded test panels. With proper selection and concentration of reinforcing fillers and antioxidants, vulcanizates of SiB-2 and SiB-4 rubber exhibit outstanding heat-aging resistance. They show tensile strengths of 400–450 psi and elongations of 100–150% at ambient temperature, and withstand temperatures of 800° to 900° F in air, demonstrating a useful life of 24 hours and 2 hours, respectively. In addition, they were still found to be flexible at −40° to −80° F, thus indicating a wide range of elasticity. Certain specialty applications where thermal-oxidative stability is required are already being considered.