Decanoic acid is one of the most inhibiting compounds with CO in the transformation of the most refractory sulfur compounds in gas oils. Unlike phenols compounds, decanoic acid and CO its main by-product present a strong inhibiting effect in the conversion of sulfur compounds. The effects are due to phenomena of competitive adsorption between sulfur and oxygen compounds on the catalyst surface. Furthermore, according to oxygenated molecules, the impact on both transformation pathways (HYD and DSD) mainly involved in HDS of gas oils is not the same. Decanoic acid and CO have a greater impact on the DSD way involved in the transformation of DBT than in HYD way involved in the transformation of 46DMDBT. These results confirmed that these two reactions require two different sites located in sulfur and metal edges of the catalyst. (c) 2013 Elsevier B.V. All rights reserved.