The electromechanical constants and polarisation moments of KTaO 3 have been determined for samples of varying size. The polarisation moments exhibit a large size dependence, and this effect is interpreted in terms of space charge, which is formed by ionisation of traps. Space-charge fields are shown to arise spontaneously and are also induced by external fields. Large external fields have been observed to alter the electro-mechanical response irreversibly. This observation is explained in terms of electron tunnelling between shallow traps and rectification of the metal-dielectric Schottky barrier.