The role of minor elements in the development, intensity and composition of slag islands visible on the weld bead was investigated. Thirty-one very low sulphur heats of ferritic stainless steel EN 1.4509 (“441”, S43940) were autogenously gas tungsten arc welded using multiple welding scenarios. The welds were analysed by using conventional metallography and scanning electron microscopy. Furthermore, the level of penetration was examined as the phenomena are connected to each other. There were two types of slag: (1) slag associated with arc ignition comprising mainly Al2O3-rafts and Al2O3-rich Al2O3-CaO, which was promoted by the presence of aluminium in the steel and high arc voltage, and (2) slag spots comprising TiO2-rich TiO2-CaO-Al2O3 with less than 40 % Al2O3. The extent of slag spots increased with increasing calcium and titanium in the steel but decreased with increasing aluminium. Calcium and titanium intensified the slag island formation and severely impaired penetration. Silicon had a detrimental effect on weld penetration but had no effect on slagging. Aluminium had a decisive role in the formation of ignition slag, but had no clear effect on slag spot formation or penetration. No statistically significant effect of sulphur or oxygen on weld penetration was found for the ranges studied.