The partial pressure of components in the gas phase in the sublimation process of SiC is of special interest for a better understanding of the Lely-crystal growth process. It is known that at thermodynamic equilibrium the gas phase of SiC has a complex composition. We have sublimated 6H-SiC and 3C-SiC under equilibrium conditions in a tantalum Knudsen cell by resistance heating. Corresponding measurements with a carbon furnace show so many contaminants that no results for the composition of SiC gas phase can be evaluated. SiC powder of different polytypes (3C-SiC and 6H-SiC), different grain size (less than or equal to 7 mu m, 40-60 mu m) and different Si/C ratio (addition of elemental silicon to the SiC powder) was evaporated in the temperature range 1900 to 2200 degrees C. The beam of the sublimated Si-C compound molecules was analyzed with a quadrupole mass spectrometer. Different Si-C molecules were identified by the ratio of the partial pressures of their natural isotopes. Si, SiC2, and Si2C were the dominant species. The heats of sublimation for the different molecules were evaluated using the Clausius-Clapeyron equation.