The pyrochlore microdomain formation in the fluorite lattice was investigated by means of electron microscope techniques, especially for the system (TbxGd1−x)2 Zr2O7+y (0 x 1; 0 y < 0.25). Specimens with x> 0.2 or specimens with x 0.2 which were quenched from temperatures above the order-disorder transition temperature show diffuse scattering in the electron diffraction images, remarkably similar to diffuse scattering observed for other defect fluorite oxides. The diffuse scattering is discussed in terms of the formation of a small basic cluster in the fluorite lattice. Annealed specimens with x 0.2 show pyrochlore diffraction spots and microdomain formation. The domains have an average diameter ranging from a value smaller than 10 nm for x = 0.2 up to 100 nm for Gd2Zr2O7. The domains grow in a disordered (fluorite) matrix and finally form antiphase boundaries. The transition from the basic cluster to a well-developed microdomain is not yet clear. The observed microdomain structure is used to explain the results of oxygen ion conductivity experiments.