Permeation measurements under open-circuit and short-circuit conditions combined with cell-voltage measurements and two probe ac impedance spectroscopy measurements were carried out in order to verify the conduction properties of La2Mo2O9 that recently has been considered a good oxygen ion conductor. As a result, a densified sample of La2Mo2O9 shows conductivity values nearly three orders of magnitude lower than those reported in literature. In addition, the electronic conduction properties of the material were characterized by determining the parameter p(circle minus) as a function of temperature. According to these findings, La2Mo2O9 does not satisfy any criterion for a useful oxide solid electrolyte. (C) 2003 Elsevier Ltd. All rights reserved.