The perovskites Pb(Sc1/2Nb1/2)O3 and Pb(Sc1/2Ta1/2)O3 can be obtained in a disordered state by means of a synthesis at high temperature (1250 to 1500°C). Ordering of the trivalent and pentavalent ions occurs during annealing at lower temperatures. The kinetics of this ordering process are investigated by means of X-ray diffraction, dielectric polarisation, and DSC measurements. The ordering process proceeds via a nucleation-growth process and the average diameter of the ordered domains as a function of the annealing time is determined by X-ray line broadening. Disordering experiments are carried out on Pb(Sc1/2Nb1/2)O3 and an order-disorder transition temperature of 1210°C is yielded.