Congruently grown LiNbO3 single crystals show both a high oxygen and lithium ionic conductivity at temperatures above 500 degreesC. The high oxygen ionic conductivity can be understood in terms of a certain amount of oxygen vacancies already present in congruently as-grown LiNbO3 single crystals. The electrocoloration phenomenon in LiNbO3 single crystals is due to a process where Lithium ions and electrons are injected by a double charge mechanism into LiNbO3. Optical and electrical properties of electrocolored LiNbO3 crystals were investigated. The motion of stoichiometric domains (regions with different stoichiometry) through a LiNbO3 crystal from one electrode to the other is studied and described in terms of electrodiffusion of the ions and electrons. (C) 2000 Published by Elsevier Science B.V.