The structures of Ni and Mo in dried, but not calcined, oxidic NiMo/SiO2 hydrotreating catalyst precursors and in the corresponding impregnation solutions were analyzed by means of UV-VIS, Raman, infrared, and X-ray absorption spectroscopies as a function of the impregnation order of the Ni and Mo salts and of the addition of the chelating agent nitrilotriacetic acid (NTA) to the impregnation solutions. The Ni ions in the impregnation solutions which contained Ni, Mo, and NTA, and in the corresponding dried catalyst precursors, were preferentially complexed with NTA. As a consequence of the Ni:Mo:NTA = 0.5:1:1 molar ratios, Mo was present in about equal amounts as the [MoO3(NTA)](3-) complex and as well-dispersed molybdates, grafted through Si-O-Mo bonds to the support. In NiMo/SiO2 samples prepared by sequential impregnation with the Mo salt and Ni in the absence of NTA, all Mo was present as well-dispersed polymolybdate grafted on the support after drying only without calcination. When the impregnation order was reversed, Ni(OH)(2) crystallites were formed on the surface of SiO2, onto which MoO42- ions attached during the second impregnation.