Increasing iron content in apatite-type La9.83Si4.5Al1.5-yFeyO26+delta(y = 0.5-1.5) leads to increasing unit cell volume, fraction of Fe4+, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973-1223 K in air, are in the range 0.986-0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10(-2) Pa to 70 kPa, confirm that under oxidizing conditions the ionic Fe3+ conduction in Fe-substituted Lag(9.83)(Si,Al)(6)O26+delta apatites is dominant. Due to stabilization of Fe3+, substantially worse transport properties are observed for A-site stoichiometric La10Si4Fe2O26, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe4+ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La10-x,(Si,Al)(6)O26+delta ceramics, calculated from dilatometric data in air, are 8.9 x 10(-6) to 9.9 X 10(-6) K-1 at 300-1250 K. (C) 2004 Elsevier Ltd. All rights reserved.