A new high-temperature elastomer, SiB-2, has been investigated by stress relaxation, modulus-temperature, and volume--temperature techniques. SiB-2 was found to be more stable than a related elastomer, radiation-cured silicone rubber, having about twice as long as a chemical relaxation time at 250°C. Possible mechanisms to account for this increased stability are discussed. At low temperatures, Tg for SiB-2 was estimated at –34°C., which compares well with Ti = −30°C. for this polymer. By comparison, SiB-3 has Ti = −60°C., while phenyl-modified SiB-4 was found to have Ti = −25°C. Tm for SiB-2 was estimated to be + 56°C.