Electrical conduction properties of complex perovskite-type oxides in the (La0.5Sr0.5)(Mg0.5+yNb0.5-y)O3-delta (y = 0.02-0.06) series at intermediate-high temperatures were investigated; introduction of protons by hydration of oxide-ion vacancies was expected by increasing the Mg/Nb) ratio from unity. The conductivity depended oil y and a maximum conductivity was obtained at y = 0.04: sigma = 4.9 x 10(-6) S cm(-1) at 400 degrees C in wet H-2 atmospheres. From electromotive force measurements of hydrogen and water vapor concentration cells, electrical conduction in wet H2 atmospheres can be attributed to ionic conduction, and proton conduction is dominant below 700 degrees C. Unlike other perovskite-type proton conductors, (La0.5Sr0.5) (Mg0.54Nb0.46)O3-delta was stable in CO2, atmospheres even in the low-intermediate temperature region due to dilution of reactive strontium by lanthanum. (C) 2008 Elsevier Ltd. All rights reserved.