The oxygen ionic conductivity of apatite-type La9.83Si4.5Al1.5-yFeyO26+/-delta (y = 0- 1.5), La10-xSi6-yFeyO26+/-delta (x = 0- 0.77; y = 1-2), and La7-xSr3Si6O26-delta (x = 0-1) increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for La9.83Si4.5Al1.5-yFeyO26+/-delta and La10Si5FeO26.5, and vary in the range 0.96-0.99 for other compositions. Doping of La-9.83(Si, Al)(6)O-26 with iron results in an increasing Fe4+ fraction, which was evaluated by Mossbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La-10(Si, Fe)O26+/-delta apatites stabilize the Fe3+ state. Among the studied materials, the highest ionic and electronic transport is observed for La10Si5FeO26.5, where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p (O-2) leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low p(O-2), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) x 10(-6) K-1 at 300-1250 K. (C) 2004 The Electrochemical Society.