The interaction between Ni2+ and Mg2+ species in Ni/MgO catalyst precursors prepared by impregnation of MgO with aqueous solutions of Ni(NO3)(2).6H(2)O, and the mechanism that produces Ni2+ cations of very low reducibility after thermal treatments of the samples at low temperature are studied by temperature programmed methods (TPD, TPR) and DRIFTS and EXAFS spectroscopies. Results clearly demonstrate that during the impregnation and mild drying steps (T less than or equal to 120 degrees C) a bulk reaction between MgO and the aqueous Ni2+ solution yields a mixed hydroxide, where Mg2+ and Ni2+ cations occupy the same type of positions in a M(OH)(2) lattice with the brucite structure. NiO/MgO solid solutions are formed by decomposition of this mixed hydroxide after thermal treatments in the 250-400 degrees C range.