Deposition rate and droplet temperatures in Gas metal arc welding of two aluminum alloys ER1100 and ER 5183 was analyzed. ER5183 tends to have higher wire feed speed and a lower droplet temperature in comparison to ER1100. It is generally considered that the higher resistivity of Mg in ER5183 causes the high deposition rate. But Joule heating cannot explain the higher deposition rate observed in the Mg-containing alloy as commonly assumed. During welding, there are several voltage losses associated with the electrode. By performing an energy balance on the electrode stickout and the molten droplet, based on the experimental measures of the droplet temperature, it was possible to ascertain an effective anode fall voltage and likely effect of vaporization. Results suggest that the lower currents observed under the same wire feed speed for ER 5183 compared to ER1100 are more likely due to the lower droplet temperature than to the different resistivity of the alloys in the electrode extension. ER5183 has lower droplet temperature by more than 300 K in spray and transition regimes, and by about 400 K in globular transfer. Also, it is likely that the metal vapours increase the anode fall voltage.