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An experimental study on MIG-MAG welding in reverse polarity (anode wire) has been implemented to analyse the influence of the active gas type and composition on the welding process. The analysis of the arc column using optical emission spectroscopy and high speed imaging completed by µ-structural study of the electrode wire by EDS/XRD and EPMA methods have provided helpful explanations on the globular/spray mode transition depending of the active gas in the shielding gas. These results highlight the existence of an oxide layer (“gangue”) and the modification of the typology of this one in globular mode according to the active gas (Ar/CO2 or Ar/O2) probably responsible of the globular/spray mode transfer by modification of the electrical/thermal conductivities of the oxide layer. Spectroscopic analysis reveals modification by an arc constriction in Ar/O2 mixtures and linked to a more prominent drop of the electronic temperature along the arc column axis. With this active gas, analysis of the Fe I / Ar I emissivity ratio show a higher metal vapours content responsible for the temperature drop by their strong radiative emission along the arc column.