Shielding gas composition is an important parameter for successful gas metal arc welding (GMAW) and has been shown to affect the fume formation rate (FFR). The present paper compares thirteen shielding gases and their impact on FFR in spray transfer. In Ar-based mixtures, increasing CO2 had a greater impact than raising O2 on FFR. When O2 was increased in ternary mixtures, the FFR increased for Ar-5%CO2 but no discernable increase was observed for the Ar-12%CO2 mixtures. Ar-He-CO2 mixtures had the most stable FFR's. The FFR for 100% CO2 was significantly higher due to the change in weld transfer mode to globular and increased spatter. Results indicate that CO2 additions in Ar-based shielding gases are the controlling factor in determining FFR due to the effect of CO2 on welding arc characteristics. There was no obvious influence from the shielding gas on particle composition and fume particles were identified as (Fe, Mn)3O4.