The ammonium decamolybdodicobattate(III) salt: (NH4)(6)[Co2Mo10O38H4].7H(2)O (here in Co2Mo10) has been prepared and characterized. Then, their structural, spectroscopic and thermal properties have been investigated by some physical-chemical techniques as XRD, Fr-IR, Raman, DRS, SEM-EDAX,TG, DTA, and temperature programmed reduction (TPR). According to these properties, some Co2Mo10/gamma-Al2O3 catalysts have been prepared by equilibrium impregnation of the support with aqueous solution of ammonium salts. The Co2Mo10 isothermal adsorption on gamma-Al2O3 at room temperature has been also investigated in the Mo range of 15-150 mumol Mo/ml. The hydrotreating (HDS of thiophene and HYD of cyclohexene) catalytic activity has been compared to both gamma-Al2O3 supported Anderson phase [(NH4)(3)[CoMo6O24H6].7H(2)O (herein CoMo6)] and Co-Mo conventional catalysts. The improving of activity for the Co2Mo10 and CoMo6 based catalysts is related to the heteropolyoxometalates adsorption process as to preparative conditions; whereas, the different activity between heteropoly-systems are undoubtedly related to the particular structure of the Co2Mo10 heteropolyanion. (C) 2002 Elsevier Science B.V. All rights reserved.