By means of transmission electron microscopy (TEM) the structure of chemically ordered regions in Zr-doped Pb(Mg1/3Ta2/3)O-3 has been studied. Large coherent ordered domains were obtained by the addition of 5 mol% PbZrO3. The results obtained by conventional, analytical, and high-resolution TEM are consistent with the random-site description of the B-site order, the so-called random-layer model. Image simulations showed that a careful selection of the imaging conditions for high resolution transmission electron microscopy is necessary in order to determine the different B-site occupancies. By comparison of simulated with experimental images it was possible to directly distinguish between the different ordering models. (C) 2004 American Institute of Physics.