Shielding gas is an important parameter in Gas Metal Arc Welding (GMAW). Nevertheless, the influence of shielding gas on particle composition and size distribution of welding aerosols is not clearly understood. Increasing the O2 or CO2 content of Ar-based shielding gases resulted in an increase in the average particle size. For binary and ternary Ar, CO2, O2 mixtures, increasing CO2 had a greater impact than raising O2 on particle size. Variations in ArHeCO2 mixtures had the least influence. For 100% CO2, the particle size distribution was altered significantly due to the change of the weld transfer mode to globular and particle size coarsened with increasing arc voltage. Shielding gas composition had no observable influence on particle composition and only a slight variation of composition with particle size was observed. Particles were identified as (Fe,Mn)3O4 with trace additions of Si.