High purity high molecular weight poly (phenylsilsesquioxane) (PPSQ) has been synthesized and specimen films prepared by a solvent casting method. The decomposition temperature of the material is 550°C in air. A PPSQ specimen cured at 350°C for 60 min has a weight loss of 1% when held at 460°C for 60 min. A specimen initially dried at 200°C for 30 min then held at 460°C loses twice as much. The main constituent of the volatile material from PPSQ pyrolyzed at 500 °C for 60 min is benzene, which is 5.7% of the specimen weight. The tensile strength for the cured specimen is 8.0 × 109 dyne/cm2 at 25 °C, and 5.5 × 109 dyne/cm2 at 250°C. The strength of the dried specimen is 4.0 × 109 dyne/cm2 at 250°C and about one half that at 25°C. The thermal expansion coefficient for the cured specimen is 11−14 × 10−5 /°C over a temperature range from 5°C to 350°C. Residual stress is 2.8 × 108 dyne/cm2 at 25°C and one order lower at 350°C.