Abstract. The features of a combustion with elementary fluorine for the case of compact SiC ceramics and model substances for boron containing ceramics (H3BO 3 and Na2B407) were investigated with the aim of their decomposition and analysis. On-line detection of the gaseous decomposition products by quadrupole mass spectrometry using electron impact ionisation was studied. Limitations by blanks and transport interferences were investigated. Standard addition as well as the isotope dilution technique were used for calibration in the case of B, C and W at the trace and major component level.