We performed pyrolysis of SiCNAlH and SiCNYOH nanopowder precursors under a reactive atmosphere (Ar/NH3/H2) with various compositions of ammonia (NH3) and dihydrogen (H2) to diminish C content, which is deleterious for thermal stability and sintering of the powders. This paper continues a previous work on the fabrication of an Si3N4/SiC composite without free C by studying the effect of H2 on the C/N atomic ratio of the powder. We studied the influence of the nature of the gaseous mixture (Ar/NH3/H2) on the powder composition. Elemental analysis showed that the introduction of H2 in the pyrolysis atmosphere limited the decomposition of NH3 and allowed for control of the C/N ratio. This behaviour can be explained by the structural evolution observed by 29Si NMR spectrometry but also by Fourier transform infrared and Raman spectroscopy. An Si3N4/SiC composite, with traces of free C, was obtained after post-pyrolysis heat treatment of the powders synthesized with 10 wt.% of H2 and 25 wt.% NH3.