In order to better understanding the effect of shielding gases in the volume fraction of δ ferrite in welded deposits through GMAW, the microstructures of four welded joints of austenitic stainless steel produced by the MIG/MAG process with different shielding gases were studied. The deposits were produced using the same welding electrode ER309Land welding parameters, but different shielding gases from pure argon to mixtures with increasing contents of CO2. Each of the welding deposits were produced with 100% Ar, Ar+2% CO2, Ar+4% CO2 and Ar+20% CO2. The chemical compositions and the variation of the volume fractions of δ ferrite in the deposits was measured. There was an increasing pickup of carbon and decreasing volume fraction of δ ferrite in the all weld metals produced using shielding gases with increasing concentrations of CO2.The results confirm that carbon is a strong austenite stabilizer in austenitic stainless steels. Complementary techniques of microstructural analysis were used, such as optical emission spectrometry, optical microscopy and quantitative image analysis.