Complete reduction of the metal phase in cobalt-containing catalysts is often difficult to reach and can be promoted by a small amount of noble metals. Because these costly promoters are introduced in a very low quantity, their speciation and their chemical interaction with cobalt have seldom been studied in detail. We present here a time-resolved in situ X-ray absorption spectroscopy investigation of the speciation of ruthenium, used as a reduction promoter of cobalt, throughout the preparation of Co/SiO2 catalysts. In the cobalt(II) nitrate impregnation solution, ruthenium is detected as hydrated Ru(OH)4 short-chain oligomers that are deposited on silica upon drying. Co3O4 forms during calcination in air and catalyzes the elimination in the gas phase of 60% of Ru. The addition of a polyol, sorbitol, in the impregnation solution stabilizes the whole of Ru on the catalyst. Upon calcination, Ru(IV) ions are inserted inside Co3O4 nanoparticles. Reduction of the oxidic phase takes place in two distinct steps, at approximately the same temperatures regardless of the ruthenium content: first to Ru(III)-containing CoO nanoparticles (Ru ions modifying the intrinsic electronic properties of the oxidic nanoparticles); then to bimetallic Co nanoparticles containing Ru(0) atoms, via an autocatalytic process. Ru loadings as low as 0.2 wt % are sufficient to afford complete reduction of cobalt. Close association between Ru and Co from the beginning of the synthesis is thus necessary for a maximum promoting effect