Molecular-scale understanding of metal impregnation on a porous oxide support is a key step toward the development of optimized (and cost ,effective) catalytic materials with enhanced activity, selectivity and time on stream stability. In the present contribution in situ ATR-IR and ex situ spectroscopic measurements (XPS, UV-vis DRS) have been performed in order to describe the influence of three different oxygenated Organic molecules (acetate, citrate, and nitrilotriacetate) on cobalt sorption on a conventional gamma-alumina support. Variation of the preparation conditions has been dime by changing the pH of adsorption, and the nature of additives (organic Molecules) as well as their concentrations. Different sorption mechanisms and Cobalt speciation have been evidenced during in situ ATR-IR measurement through comparison with reference spectra of pure Co and organic additives solutions. More specifically, citrate and nitriloacetate additives leads to the formation of ternary surface complexes (alumina ligand cobalt) preventing Co precipitation at high pH and leading to a higher Co dispersion at the impregnation step than for the ligand-free system. However, acetate does not sorb on alumina and cannot prevent Co precipitation. However, addition of acetate in the impregnation solution leads to an increase in the amount of cobalt adsorbed at high pH but induces a strong decrease of cobalt sorption at lower pH