The conductivity of La0.9Sr0.1Sc0.9Mg0.1O3 has been studied by impedance spectroscopy in controlled atmospheres. The material was found to be a mixed conductor with p-type conduction at high oxygen partial pressures and a combined proton and oxide ion conductor at low oxygen partial pressures. At temperatures below 800 degrees C and low partial pressure of oxygen, proton conduction was dominant. Above this temperature, the ionic conductivity is dominated by oxide ion transport. The protonic transport number was estimated from the conductivities measured in dry and in water-moisturised gas. An isotope effect was observed using D-2/D2O in place of H-2/H2O. The oxygen transport number was estimated from this effect and from measurements in a two-chamber electrochemical cell. (C) 1999 Elsevier Science B.V. All rights reserved.