The pyrolytic conversion of a polyvinylsilazane precursor to Si-C-N ceramics is studied using solid-state NMR. 13C and 29Si magic angle spinning and 1H static NMR experiments allow us to investigate the structure of the disordered multiphased intermediates formed during the pyrolysis. The Si environments are quantitatively analyzed by 29Si MAS NMR spectra simulations. The relative abundance of SiHCN2, SiC2N2, SiN3C and SiN4 sites at different temperatures are used to calculate elemental compositions that characterize the Si-containing phase. The comparison of these compositions with those obtained from chemical analyses shows evidence of the presence of a free carbon phase.We have also determined the H and C contents in the free carbon phase.