The effect of various impregnation procedures on the structures of oxidic phosphorus-containing NiMo/Al2O3 and CoMo/Al2O3 catalysts was studied by quantitative P-31 and Al-27 solid State NMR. NH4H2PO4 was used as the phosphorus source, since it reacts less strongly with the alumina support than does H3PO4, The presence of paramagnetic Co and Ni ions induces strong line broadening and a signal loss in the P-31 and Al-27 NMR spectra which can be used to deduce the distribution of Co and Ni over the catalyst surface. Large differences in this distribution existed, depending on the paramagnetic ion and the impregnation procedure. The addition of phosphorus led mainly to the formation of Co-Mo-P compounds in samples containing cobalt, whereas the creation of an aluminum phosphate phase was favored in nickel-containing catalysts, Are impregnation procedure in which Ni or Co and phosphorus are separately deposited on the catalyst surface also promoted AlPO4 formation, so that even in the samples containing cobalt some aluminum phosphate is observed. On the other hand, coimpregnation of Co or Ni and phosphorus resulted in catalysts which contained little aluminum phosphate. Possible consequences for hydrotreating reactions are discussed.