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 La 2 Mo 2 O 9 that recently has been considered a good oxygen ion conductor. As a result, a densified sample of La 2 Mo 2 O 9 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 as a function of temperature. According to these findings, La 2 Mo 2 O 9 does not satisfy any criterion for a useful oxide solid electrolyte.