We have synthesised the new protonated layered perovskite H2La2/3Ta2O7 which is related to the Ruddlesden-Popper family. This compound is obtained by ionic exchange starting from Li2La2/3Ta2O7 maintained in dilute HNO3 at 60 degrees C. Thermal X-ray diffraction and DTA/TGA revealed interesting dehydration properties with formation of a layered anhydrous phase leading at higher temperature (1550 degrees C) to La1/3TaO3. This latter compound exhibits the original lanthanum ordering expected similarly to that of the Li form, while at 900 degrees C a metastable form, presenting a disordered La distribution, is observed. (c) 2005 Elsevier Ltd. All rights reserved.