The effect of activation conditions and the hydrodesulfurization (HDS) process on the structure of Ru/MgF2 prepared from RuCl3 : nH(2)O precursor catalysts was studied. During the first 6 - 7 h of the HDS process significant differences in activities were detected between samples pretreated in 50% H2S/H-2 and in 50% H2S/He. During this transient time, the Ru/MgF2 catalyst pretreated in 50% H2S/H-2 increased in activity while the activity of that pretreated in H2S/He decreased. XRD, TPR and quantitative analysis of sulfur content revealed that directly after activation in H2S/He crystalline ruthenium sulfide is the main component of the surface, whereas after activation in H2S/H-2 both RuS2 and metallic Ru are present on the surface. These differences decrease during the thiophene HDS process. Despite the different pretreatment conditions, the composition of the surface tends to "equilibrate'', which results in similar activity after prolonged reaction time.