GaPO4 crystals were grown from solutions of aqueous phosphoric acid. A critical factor influencing the high-temperature behaviour of the material is its content of -OH groups, which was determined by FT-IR spectroscopy. From an Arrhenius plot of the electrical conductivity of platinum electroded crystals, a single activation energy of 1.68 +/-0.07 eV to temperatures of about 950 degrees C was derived irrespective of the source of the crystals. A conduction model based on proton migration via a hydrogen bridge bond between an OH group and an adjacent oxygen ion is proposed. (c) 2006 Elsevier B.V. All rights reserved.