We have studied the formation of lead-magnesium-niobate Pb(Mg1/3Nb2/3)O-3 perovskite powder produced by high-energy milling of the constituent oxides. By applying a low ball-impact energy and a low ball-impact frequency we were able to identify the reaction sequences. The crystal structure and the amount of crystalline and amorphous phases in the powder were determined using a Rietveld refinement. The morphology, structure and chemical composition of the powder were investigated by transmission electron microscopy. The surface composition, possible contamination and the chemical states of the elements were analyzed by X-ray photoelectron spectroscopy. In the early stage of milling the constituent oxides comminute, refine to nanosize dimensions and become amorphous. In the second step the Pb(Mg1/3Nb2/3)O-3 nucleates from the amorphous regions where the stoichiometry corresponds to the perovskite. The perovskite phase nucleates and subsequently grows over the course of the milling. Simultaneously, the pyrochlore phase nucleates from the Pb- and Nb-rich regions; however, its nucleation and recrystallization stop after a certain milling time. The powder mixture consists of nanosized Pb(Mg1/3Nb2/3)O-3 particles and an amorphous phase when milled for 94 h. No traces of contamination from the milling media were detected.