The present work deals with mullite (Al2O3.2SiO2) as a potential material for application as solid electrolyte in high-temperature electrochemical devices. The structure and preparation of mullite and the design of mullite-based oxygen sensors are described. The predominantly ionic conduction of mullite is characterized by means of the parameter P(e’) (t(ion) = 0.5) of electronic conductivity and the temperature dependence of the ionic transference number t(ion). It is confirmed that mullite features ionic conductivity superior to that of the usual CaO-stabilized ZrO2 solid electrolytes at temperature from 1400 to 1600-degrees-C.