The electrochemical properties of dense La9Sr1Si6O26.5 pellets were studied. High conductivity values were measured (8.8 X 10(-3) S cm(-1) at 700 degrees C) for this pure ionic conductor. Semipermeability measurements allowed measuring an electronic transport number of 5 X 10(-5) at 700 degrees C under air. These results prove that La9Sr1Si6O26.5 is a pure ionic conductor. Ageing tests of this compound during one month showed a stable behaviour of the conductivity under air and a slight decrease of it under humidified atmosphere. (c) 2007 Elsevier B.V All rights reserved.