The incorporation of fluorine into porous silica by gas phase doping with hexafluoroethane C2F6 in N2 was studied using thermodynamic calculations and complementary fluorination experiments up to 1300 °C. With online FTIR analysis of the gas phase, the main products SiF4, CO2, CO, and CF4, and the traces H2O, HF, Si2F6O, and COF2 were detected and three consecutive kinetic phases deduced: the unique sorption phase, a combined etching and fluorination phase, and the thermal decomposition of C2F6 accompanied by a deposition of solid carbon. The chemical species were connected to a framework of chemical reactions. Carbon deposition and mass loss by etching are principle problems of this fluorine precursor, that cannot be completely avoided. According to the origin, three types of carbon can be distinguished. Because of its high thermodynamic stability, SiF4 is a key product of etching.