Replacement of divalent Sr2+ ions by monovalent K+ ions in La0.9Sr0.1Ga0.8Mg0.2O2.(85) (LSGM) decreases the electrical conductivity of LSGM and increases the activation energy to 1.42 eV. Substitution of La by substantial amounts of up to 50 atom % of Pr in LSGM yields new mixed oxide ion and electronic conductors. Substitution of 50 atom % Pr for La in the In-analog of LSGM, La0.9Sr0.1In0.8Mg0.2O2.85 (LSIM), does not increase the electrical conductivity. Among the investigated oxides, La0.4Pr0.4Sr0.2In0.8Mg0.2O2.8 exhibits the lowest activation energy of 0.44 eV and the highest electrical conductivity of 3.17 x 10(-5) S/cm at 200 degreesC. Oxygen partial pressure dependence of the electrical conductivity reveals that LSIM and Pr-substituted LSIM perovskites are mixed oxide ion and p-type electronic conductors at high oxygen partial pressures. The electronic conductivity increases with increasing Pr content in LSIM. Open-circuit voltage measurements employing La0.45Pr0.45Sr0.1Ga0.8Mg0.2O2.85 as separator in the galvanic cell H(2)parallel to air show that the average transference number (t(O)(2-)) for oxide ion conduction is 0.78-0.90 in the temperature range 450-650C. Powder X-ray diffraction data reveal that LSIM and Pr-substituted LSIM perovskites are not stable at low oxygen partial pressures of about 10(-22) atm, while Pr-substituted LSGM retains the cubic perovskite structure. Accordingly, the new materials reported here may find application as electrode materials for solid oxide fuel cells and oxygen sensors.