Doubly doped LaErO3 ceramics, La0.9Ba0.1Er1-x Mg (x) O3-alpha (x = 0.05, 0.10, 0.15, 0.20), were synthesized by solid-state reaction method and characterized by X-ray diffraction (XRD). The samples have a single orthorhombic perovskite-type structure. The conduction behavior was investigated using various electrochemical methods including AC impedance spectroscopy, gas concentration cell, isotope effect of hydrogen, and hydrogen electrochemical permeation (pumping) in the temperature range of 500-1000 A degrees C. The results indicated that specimens were pure ionic conductors under low oxygen partial pressure (about 10(-7)-10(-20) atm) and mixed conductors of proton, oxide ion, and electron hole under high oxygen partial pressure (about 10(-5)-1 atm). The pure ion conduction of the ceramics in hydrogen atmosphere was confirmed by electromotive force method of hydrogen concentration cell, and the observed emf values coincided well with the theoretical ones. The conductivity in H2O-Ar atmosphere was higher than that in D2O-Ar atmosphere, exhibiting an obvious isotope effect and proton conduction in water vapor containing atmosphere. It has been confirmed by electrochemical hydrogen permeation (hydrogen pumping) experiment that the ceramics were mainly proton conductors in hydrogen containing atmosphere. Whereas in dry oxygen-containing atmosphere, observed emf values of the oxygen concentration cell were far lower than the theoretical ones, indicating that the ceramics were mixed conductors of electron hole and oxide ion.