The phase diagram (T-x plot) of thermally depoled tetragonal PLZT materials has been investigated by means of X-ray diffraction, Capacitance and DSC measurements. Materials with a low La content show a classical Fet↔PEc transition. This transition has first or second order character, dependent on the Zr/Ti ratio. Materials with a medium and high La content show a diffuse transition from the cubic high temperature phase to a tetragonal, so called β phase. This β phase has no clear FE properties and a spontaneous transition to the FEt phase on lowering the temperature only takes place for materials with a medium La content. For materials with a high La content a spontaneous βt→ FEt transition is not observed.