Phenyl acetylene terminated poly(carborane-silanec) (PACS) was synthesized by the couple reaction of methyldichlorosilane with 1,7-dilithio-m-carborane and lithium phenylacetylide. The structure was characterized using FTIR, H-1-NMR, C-13-NMR, Si-29-NMR, and gel permeation chromatography. PACS exhibits solubility in common organic solvents. Thermal and oxidative properties were evaluated by thermogravimetric analysis (TGA), Thermoset exhibits extremely thermal and oxidative property and TGA curves show that the temperature of 5% weight loss (Td(5)) is 762 degrees C and char yield at 800 degrees C is 94.2% in nitrogen. In air, surprisingly, both Td(5) and char yield at 800 degrees C show slight increase, which is greater than 800 degrees C and 95.6%, respectively. After pyrolysis, the char has no additional weight loss up to 800 degrees C in air. (c) 2007 Wiley Periodicals, Inc.