Polysilazanes have been described as excellent polymeric precursors to amorphous silicon carbonitride (SiCN) glasses. In this work, the synthesis, thermal cross-linking and pyrolysis of a polycyclic silazane network precursor for a SiCN glass, were studied. This polymeric precursor was prepared by free radical polymerization of 1,3,5-trimethyl-1 ' ,3 ' ,5 ' -trivinylcyclotrisilazane, using dicumyl peroxide as radical initiator. The polymer-to-ceramic conversion was investigated by 2 9 Si and 1 3 C magic angle spinning nuclear magnetic resonance (MAS NMR) and infrared spectroscopies, X-ray diffraction and by simultaneous thermogravimetric and differential thermal analyses. Based on the results the polymeric precursor can be characterized as a polycyclic silazane network, containing aliphatic carbon segments produced by the vinyl polymerization. 2 9 Si and 1 3 C NMR data of this polymeric precursor heated at different temperatures indicated the occurrence of side-reactions in addition to the vinyl polymerization. The SiCN glass was obtained as an amorphous solid up to 1400 o C. Above this temperature, the crystallization process of the ceramic product was promoted by rearrangement of the different Si sites in the amorphous network. Heat treatment at 1600 o C, under N 2 atmosphere, produced a ceramic containing 85% silicon carbide (SiC) and 15% silicon nitride, determined by 2 9 Si MAS NMR in relation to the total silicon sites, and under argon atmosphere, SiC.