The local structure of the interlamellar lanthanide ions in montmorillonite upon heating is studied. The analysis of the EXAFS data has been carried out using two model environments, that of La3+ aquocomplex and that of the lanthanum oxide. The information thus obtained is crucial in elucidating the intercalation mechanism and demonstrates the important role that XAFS techniques can play in the knowledge of the location and environment of the exchangeable ions in layered silicates. Dirección General de Investigación Científica y Técnica de España PB89-0642