Fine-grained powders of excellent homogeneity represent a requirement for high performance materials based on silicon nitride and silicon carbide. Such powders can be produced by pyrolysis of organometallic precursors. The investigated silicon-carbonitride powders were prepared by gas phase-pyrolysis of disilazane or oligosilazanes under different pyrolysis conditions. On-line coupling of thermogravimetry (TG) with evolved gas analysis (EGA)/mass spectrometry (MS) was applied to characterize the behaviour of powders during annealing processes. The results show clearly the influence of pyrolysis conditions and post-treatment steps on the surface properties of powders.