This article is devoted to the computation of equilibrium compositions of the shielding gases in gas metal arc welding (GMAW) Ar, CO2, and 82%Ar-18%CO2 as well as their mixtures with Fe metal vapour at atmospheric pressure between 3 000 and 25 000 K. The above gases were treated as a type of Ar-CO2-Fe mixture with 39 particles. These particles were divided as two categories: 5 basis particles and 34 non-basis particles. The particle number densities of non-basis particles were expressed by that of basis particles based on chemical equation, thus reducing the unknowns and the computation. Finally, the conservation equations were solved by Newton’s method to obtain the number density of each particle. The results indicate that primary and secondary ionization occur in pure argon with the increase of temperature, the dissociation of molecules of CO2, O2 and CO at low temperature (T＞8 000 K) occur in pure CO2 besides the ionization at high temperature, and both ionization and dissociation exist in the 82%Ar-18%CO2 mixture. It also found that the addition of Fe can increase the electron number density, especially at temperature below 15 000 K. The determination of chemical compositions built a solid foundation for the calculation of radiative properties of GMAW arc plasmas and for the spectroscopic measurement of iron vapour concentration in arcs.