The present study examines the effect of sonication on the structure and on the thermal behaviour of a well ordered kaolinite. XRD data indicated that sonication produces an increase of the translation disorder of kaolinite layers as well as delamination and particle-size reduction along the b-axis. As a consequence of this particle-size reduction the surface area increases sharply with sonication time from 8.5 to 83 m(2)/g, after 20 h of treatment. When sonication time increases, the original dehydroxylation effect shifts to lower temperatures, is narrowing and decreases in intensity at the same time that the exothermic effect, previous to the mullite formation shifts to lower temperatures and notably broadened. (C) 2003 Elsevier Science B.V. All rights reserved.