Hydrodesulfurization (HDS) catalysts prepared by the incipient wetness impregnation method with Co-Mo-P based solutions were characterized with various physical techniques and their efficiencies in HDS of thiophene were evaluated. Depending on the Co-Mo-P precursors used to prepare the impregnating solution, different heteropolyanions (HPAs) were characterized. In conventional solutions, P2Mo5O236- or P2Mo18O626- type entities were evidenced. Preparations with Keggin type HPAs were also studied (PMo12O403-) and an original method of preparation which allows the deposition of alumina on reduced Keggin type HPAs (PMo12O407-) is described. If the non-reduced HPAs (whatever their structure) are decomposed on alumina, the reduced ones could be partially preserved even after calcination, leading to the synthesis of more efficient catalysts.