The nano-scale phase separated domain structures of Pb(Mg 1 / 3 Ta 2 / 3 )O 3 (PMT) relaxors are shown to be metastable and responsive to high temperature heat treatments. For pure PMT the 2-3-nm ordered domains in an as-sintered sample coarsen to approximately four times their original size (~8 nm) after thermal annealing at 1325 o C for 48 h. The alterations in the domain coarsening and degree of cation order are greatly enhanced in solid solutions of PMT with PbZrO 3 (PZ). For 5-15 mol% substitutions fully 1:1 ordered microstructures comprised of ~40-nm domains were stabilized after the same type of annealing. Quenching experiments revealed that the enhancements in the degree of order were related to a thermodynamic stabilization of the 1:1 order by the Zr cations. Models based on the random site description gave the most consistent interpretation of the cation ordering. The retention of a relaxor ferroelectric response in all the 1:1 ordered large domain PMT-PZ ceramics, indicates that randomness on one of the positions of the ordered structure, and not the chemical domain size, is the critical factor in inhibiting normal ferroelectric coupling.