The introduction of a glycol-type additive in hydrotreating catalysts is an efficient procedure to improve catalytic activity. Nevertheless, controversial explanations about the activity enhancement mechanism exist in the literature. This may be due to different catalyst preparation procedures, different location of the additive impregnation step, or simply because several phenomena are implied in this improvement. The aim of this work is thus to rationalize the roles of these additives with respect to (i) species present in the impregnation solution as well as on the catalyst surface and (ii) the preparation step where the additive impregnation is performed, i.e. after drying or after calcination. Different impregnation solutions have been used containing (a) ammonium heptamolybdate or cobaltomolybdate heteropolyanions for CoMo catalysts and (b) phosphomolybdate heteropolyanion with different P/Mo molar ratio for CoMoP catalysts. Surface species have been thoroughly characterized for dried and calcined catalysts prior to and after the additive impregnation using triethyleneglycol. For all dried and calcined CoMo and CoMoP catalysts, a redissolution phenomenon has been evidenced after the additive impregnation, leading to the formation of the Anderson heteropolyanion AlMo6O24H63-. This redissolution phenomenon is however limited by the low solubility of AlMo6O24H63-. Moreover, in the case of CoMoP dried catalysts (P/Mo molar ratio >= 0.4), characterization of additive-containing catalysts evidenced PCoMo11O407- formation. Redissolution and redispersion due to the additives are thus enhanced because phosphomolybdic species have a much higher solubility than AlMo6O24H63-. Similar observations, although less pronounced, may be drawn for calcined catalysts. Indeed, a stronger precursorsupport interaction has been created during calcination. Catalysts performances were evaluated in toluene hydrogenation and activities obtained match perfectly. (C) 2007 Elsevier B.V. All rights reserved.