CVD silicon nitride (Si3N4) is typically produced from gas or liquid precursors containing nitrogen and silicon. The method using Na2SiF6(s) as silicon solid precursor to produce films/coatings, reinforcements and powders of silicon nitride by CVD has been recently proposed in the literature. In this investigation, a thermodynamic study is carried out using the FactSage Thermochemical Software and Databases, in order to explain the phenomena associated to the synthesis of Si3N4 with Na2SiF6 as solid precursor. Accordingly, CVD diagrams for Na2SiF6, SiF4, SiF3, SiF2, SiF, and Si both with N2 and NH3 are constructed using such a software. Thermodynamically Si3N4 can be produced from SiF4(g) or Na2SiF6(s) with ammonia. Although thermodynamic considerations show that Si3N4 cannot be produced with the use of nitrogen, experimental results in this investigation show that it is formed with both ammonia and nitrogen.