The conversion to ceramic of a commercial polycarbosilane (PCS) under various pyrolysis conditions has been investigated. The products of pyrolysis have been characterised by solid state Si-29 and C-13 NMR spectroscopy and X-ray diffraction (XRD). Some of the phases identified in the present study were found to differ from those reported previously, particularly in the earlier literature. Oxidation-cured PCS, when pyrolyzed up to 1400 degreesC in argon, generally produced silicon oxycarbide (SiOxCy) as the second major phase with beta -SiC as the major phase, and smaller amounts of free carbon. With increasing temperature above 1200 degreesC, the silicon oxycarbide phase decomposed to give beta -SiC. Silica (SiO2) was also found to evolve from this silicon oxycarbide phase. Loss of some of the silica, probably by reaction with carbon, was found at 1400 degreesC, possibly yielding SiO, CO and SiC. At 1500 degreesC, crystalline alpha -cristobalite was found as a minor phase with beta -SiC as the major phase and a lower amount of free carbon. Pyrolysis in vacuum leads to production and crystallization of beta -SiC at a lower temperature than required if pyrolyzed in argon flow. After pyrolysis at 1600 degrees in vacuum, the cured PCS converted to almost stoichiometric beta -SiC.