Extended thermal annealing treatments were used to modify the B-site cation order in the(1−x)PMN−(x)PSNperovskite system. Extensive 1:1 ordering could be induced in compositions withx⩾0.1.The substitution of PSN into PMN produces a large increase in the thermal stability of the 1:1 ordered phase, with the maximum disordering temperature of ∼1360&hthinsp;°C being observed forx=0.5.The order-disorder transition temperature for pure PMN was calculated to be 913&hthinsp;°C. The changes in stability could be rationalized using the random site model for the cation order. The well ordered, large chemical domain ceramics exhibited relaxor behavior up tox∼0.6,for higher values normal ferroelectric behavior was observed. Alterations in the size of the chemical domain size had no significant effect on the properties of the lower x compositions, but induced a transition to relaxor behavior forx>∼0.6.©2000 American Institute of Physics.