Three exactly alternating carborane-siloxane polymers P1, P2, and P3, have been synthesized by the polycondensation of 1,7-bis(dimethylhydroxylsilyl)-m-carborane and cyclotrisilazanes in the presence of (NH(4))(2)SO(4). The reaction shows a new and convenient synthesis of poly(carborane-siloxane)s, and the first example of stoichiometric polycondensation of cyclosilazanes with diols. The new polymers P2 and P3 could be converted into thermosets at elevated temperature through the cross-linking of pendant vinyl groups. The structures of the polymers were fully characterized. Thermogravimetric analysis (TGA) showed the superior thermal stability of the polymers and thermosets, with their char yields of over 83% both in air (800 degrees C) and nitrogen (1000 degrees C). The microstructure of the chars was studied by scanning electron microscopy (SEM). It was found that the cross-linking of vinyl groups helped in enhancing the thermal stabilities of the polymers and keeping the integrity of the chars. The results suggest their potential utility under severe thermal and/or thermo-oxidative conditions. (C) 2011 Elsevier Ltd. All rights reserved.