The perovskite La0.7Sr0.3Sc0.8Ti0.2O2.95 has been prepared and its transport properties studied over the temperature range 170-1000 degreesC in varied oxygen partial pressures. The electrical conductivity was measured by impedance spectroscopy and the four-point dc technique in atmospheres of N-2/H-2/H2O or N-2/O-2/H2O; further measurements in N-2/D-2/D2O or N-2/O-2/D2O were used to observe the conductivity isotope effect. The faradaic transport of oxygen was measured in a two-compartment cell in an atmosphere of N-2/H-2/H2O. La0.7Sr0.3Sc0.8Ti0.2O2.95 in reducing atmospheres is found to exhibit both protonic and oxide ion conductivity, while at high oxygen partial pressures, p-type conductivity is observed. The total conductivity at 800 degreesC in air with similar to1% H2O was 14 mS cm(-1). At the same temperature, the observed H/D isotope effect indicates a protonic transport number of between 0.3 to 0.4, while the measurements on the faradaic transport cell gave a value of 0.5. (C) 2003 Elsevier B.V. All rights reserved.