The ceramic Li3xLa2/3-x,1/3-2xTiO3 is used, in an all-solid-state configuration, as pH-sensor in aqueous buffer solution. This ceramic displays a pH sensitivity comparable to the one obtained with a commercial glass electrode and does not show any sensitivity to the redox potential of the solution. This is one of the remarkable properties of this ceramic pH-sensor. It is shown in this paper that the reproducibility and the behaviour of the pH-response depend on the morphology of the grain boundaries particularly on the size of the grains. A so-called 'good' pH-response is obtained with a ceramic showing big and homogeneous grains. Complex impedance spectroscopy reveals that such ceramic has a high grain boundary resistance. Furthermore, this electrochemical technique allows us to determine that the interface reaction involved in the pH detection has a time constant of the order of the second. Several assumptions such as ion exchange or acid-base reaction are proposed to explain the sensitivity of the ceramic material to the pH of the solution. (C) 2003 Elsevier Science Ltd. All rights reserved.