The incorporation of praseodymium in the apatite-type lattice of La9.83-xPrSi4.5Fe1.5O26+/-delta (x=0-6) decreases the unit cell volume, suppresses Fe4+ formation according to Mossbauer spectroscopy, and increases p- and n-type electronic contributions to total conductivity, studied by the impedance spectroscopy and modified faradaic efficiency (FE) and electromotive force (EMF) methods at 973-1223 K. The additions of praseodymia have no essential effect on the ionic transport, with an activation energy of 99-109 kJ/mol, under oxidizing conditions. Contrary to the M-containing analogue, La9.83S4.5Al1.5O26, exhibiting P(O-2)-independent conductivity at oxygen pressures from 10(-20) to 0.5 atm, the ionic conductivity of Lag(9.83-x)Pr(x)Si(4.5)Fe(1.5)O(26+/-delta) decreases on reducing p(O-2) below 10(-14) 10(-12) atm. The observed behavior suggests a presence of hyperstoichiometric oxygen, critical for the level of ionic conduction and compensated by the formation of Fe4+ or Pr4+. The ion transference numbers in air vary in the range 0.979-0.994 for La9.83-xPrxSi4.5Fe1.5O26+delta, whilst for La9.83S45Al1.5O26 the p-type electronic contribution to the total conductivity is lower than 0.5%. The average thermal expansion coefficients (TECs) of silicate-based solid electrolytes are (8.8-9.4) x 10(-6) K-1 at 300-1200 K and (14.2-15.8) x 10(-6) K-1 at 1200-1350 K. (C) 2004 Elsevier B.V. All rights reserved.