The reaction chemistry involved in the synthesis of perovskite Pb(Mg1/3Nb2/3)O-3 [Pb3MgNb2O9] was studied by the solid state reaction technique using precursor oxides as reactants. At the initial stage of the reaction process, a large fraction of PbO present in the mixtures combined with Nb2O5 and a small amount of MgO to form an oxygen-deficient pyrochlore phase with a composition Pb-1.714(Mg0.286Nb1.714)O-6.286 [Pb6MgNb6O22]. The pyrochlore phase thus formed further reacted with the remaining PbO and MgO to yield the perovskite Pb(Mg-1/3Nb(2)/3)O-3. The pyrochlore Pb-1.714(Mg-0.286Nb(1).714)O-6.286 accomodates a small amount of PbO into its lattice and forms a narrow homogeneity range which extends from the composition Pb-1.714(Mg-0.286Nb(1).714)O-6.286 [Pb(6)MgNb(6)O(2)2] to a composition Pb-2(Mg0.286Nb1.714)O-6.571 [Pb7MgNb6O23] with a corresponding increase in the lattice constant value from a = 10.586 to 10.601 Angstrom. The pyrochlore phase melts incongruently at a temperature near 1230 degrees C to yield Mg4Nb2O9 and a liquid. Below this temperature, the perovskite Pb(Mg1/3Nb2/3)O-3 coexists with the pyrochlore solid solutions. However, the compound Pb(Mg1/3Nb2/3)O-3 is not compatible with Nb2O5 and these two phases react with one another to form the pyrochlore Pb-1.714(Mg0.286Nb1.714)O-6.286 and MgO.