Phase equilibria in the PbO-MgO-Nb2O5 system have been determined at 1000 degreesC and 1 atm. A solid-solution range of the cubic pyrochlore phase was observed and the extent of that domain was determined. This solid solution equilibrates with the surrounding ternary and binary compounds to form a series of diphasic areas. The compatibility relations between all phases were determined except for a small domain where at least six lead niobate phases were stable within a very short range of compositions. The formation of pyrochlore impurities during the processing of Pb(Mg1/3Nb2/3)O-3 perovskite is discussed in relation to the phase diagram.