We review results of density functional theory calculations of the adsorption of single gold atoms and formation of sub-nanometer Au n structures (n = 2 to 5) on most stable iron oxide surfaces: hematite (0001), and magnetite (111) and (001). Structural, energetic, and electronic properties of Au n structures on both Fe- and O-rich oxide terminations are discussed. Different chemical character of the two oxide terminations is reflected in distinctly stronger binding of gold at the oxygen- than at the iron-terminated surface, and in different changes of the adsorption binding energy with the size of the Au n cluster. On the iron-terminated oxide surface the binding energy increases whereas on the oxygen-rich termination it decreases with the number of Au atoms in the structure. Upon CO adsorption on magnetite surface all Au n structures have a net positive charge and CO binds to the most cationic Au atom of a cluster. Interactions of Au n and CO with magnetite (111) show many similarities with those on hematite (0001) surface. The influence of the substrate relaxation effects on adsorption energy is also discussed.