The development of novel oxygen ion conducting solid electrolytes is of great interest for high-temperature electrochemical applications such as solid oxide fuel cells (SOFCs). This work was focused on the study of transport properties of apatite-type La10Si6−xFexO27−x/2 (x = 1–2). Single-phase apatite ceramics with density higher than 98% were prepared by the standard solid-state synthesis route. The materials were characterized by X-ray diffraction, dilatometry, impedance spectroscopy and faradaic efficiency measurements. The total conductivity and Seebeck coefficient were studied as function of the oxygen partial pressure varying in the range 10−16 Pa to 50 kPa. The ionic conductivity of apatite phases was found to increase with oxygen content. In air, the ion transference numbers of La10Si6−xFexO27−x/2 (x = 1.0–1.5) at 700–950 °C are higher than 0.99, whilst the p-type electronic contribution to the total conductivity of La10Si4Fe2O26 is about 3%. Mössbauer spectroscopy showed that the coordination of iron cations, which are all trivalent within the detection limits, increases with oxygen intercalation in the lattice. Reducing p(O2) below 10−8 Pa leads to a decrease in the ionic transport and growing n-type electronic contribution, the role of which increases with iron additions. The average thermal expansion coefficients in air are (8.2–9.9) × 10−6 K−1 at 100–1000 °C.