The ionic conductivity of the Ln(2)Ti(2)O(7) (Ln=Tm-Lu) pyrochlores is found to correlate with the degree of anti-site cation disordering in these materials. The highest ionic conductivity is exhibited by Ln(2)Ti(2)O(7) containing 3.53-4.5% Ln(Ti)+Ti-Ln anti-structure pairs. This degree of cation disordering can be achieved by heat treatment just below the melting point of the material, by mechanical activation with subsequent high-temperature firing and by doping with small amounts of cations of different sizes (as, e.g., in the case of (Yb0.91Sc0.09)(2)Ti2O7). The Ln(Ti)+Ti-Ln anti-structure pairs seem to interact with oxygen vacancies. The defect structure of Lu2+xTi2-xO7-x/2 (34.5-44 mol% Lu2O3) oxygen-ion conductors is investigated. The materials with x=0.052 (34.5 mol% Lu2O3); 0.096 (35.5 mol% Lu2O3); 0.286 (40 mol% Lu2O3) contain the whole one type of anti-site defect, Lu-Ti, and possess an ionic conductivity of about 10(-3) S/cm at 740 degrees C. The conductivity of the materials containing Lu-Ti defects is lower than that of the stoichiometric materials containing Lu-Ti + Ti-Ln anti-structure pairs. A strong correlation is found between the variable positional parameter X of 0(2) (position 480 in the structure of Lu2+xTi2-xO7-x/2,,Ti-2(-),O7-x/2 (x=0, 0.052, 0.096, 0.286) nanoceramics and the I-311/I-222 and I-331/I-222 intensity ratios in their X-ray diffraction patterns. (c) 2005 Elsevier B.V. All rights reserved.