The formation process of perovskite solid-solution 0.9Pb(Mg1/3Nb2/3)-0.1PbTiO3 (0.9PMN-0.1PT) from a stoichiometric mixture of PbO, Nb2O5, TiO2 and Mg(OH)2 by a soft-mechanochemical technique was studied. Single phase of perovskite 0.9PMN-0.1PT powder was obtained after heating at 850°C for 4 h of the mechanically activated precursor with a stoichiometric composition maintained. After milling a starting mixture for 60 min, Nb5+ was locally reduced to Nb2+ and the amount of -OH in Mg(OH)2 decreased, as confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR), respectively. Formation of precursors seems to be accomplished by preferential solid-state acid/base reactions between Nb2O5 and Mg(OH)2 under mechanical stress. All these observations consistently explain the enhanced rate of formation of perovskite solid-solution 0.9PMN-0.1 PT from the mechanically activated mixture on subsequent heating.