The stoichiometry of the wet chemical etching of silicon in concentrated binary and ternary mixtures of HF, HNO3 and H2SiF6 was comprehensively investigated. A complete quantification of both dissolved and gaseous reaction products was carried out for a variety of different acid mixtures. It could be shown that the total nitric acid consumption is directly determined by the concentration of undissociated HNO3 in the mixture and can be attributed to the consumption in subsequent reactions with increasing concentration. Furthermore, a critical minimum concentration of undissociated HNO3 of q(HNO3, undiss) ≥ 0.35 mol kg-1 could be determined, which is required to start the reaction at 20 °C with agitation, irrespective of the composition of the mixture (binary/tertiary). The simultaneous determination of the nitrogen oxides in the gas phase supports the theory that NO is the only direct reduction product of HNO3 in the reaction with Si. Furthermore, the amount of formed hydrogen is determined by both the HF and the HNO3 concentration in the mixture. For binary mixtures, the H2 formation can be quantitatively described as a function of the concentration of HNO3, HF and H2O. The most important finding from comparative investigations between binary and ternary mixtures is that the overall reaction is largely determined by the formation of the reactive intermediate HNO2 as a result of complex reaction pathways. Both the formation and the accumulation of this intermediate are determined by the water content of the etching mixture. The consumption of HNO3 and also the formation of the reaction products NOx and H2 can therefore be functionally described on the basis of the H2O content in the etching mixture, regardless of a binary or ternary mixture.