The effect of different tetravalent octahedral-site substitutions (M4+ = Zr, Tb, and Ce) on the stability of the 1:1 chemically ordered phase in Pb(Mg1/3Nb2/3)O-3 (PMN) relaxer ferroelectrics were investigated using X-ray diffractometry and transmission electron microscopy, In their as-sintered form, all the samples adopted a phase-separated microstructure that consisted of nanosized 1:1 ordered domains dispersed in a disordered matrix, However, after annealing at 1250 degrees C for 24 h, a fully 1:1 ordered microstructure with large domains (>100 nm) was stabilized in 10 mol%-terbium-doped PMN. No ordered-domain coarsening was observed in pure or zirconium-doped PMN, which indicated that the size of the solid-solution additives was critical to stimulating the growth and stability of the 1:1 order. The observation of complete 1:1 order in the terbium-doped system supports the "random-site" model description of the Pb(beta(1/2)'beta(1/2)")O-3 structure, This model also can be used to explain the insensitivity of the diffuse frequency-dependent dielectric properties to the size of the chemically ordered domains.