A kinetic study of the transformation of HGdP(2)O(7)(.)1.5H(2)O into HGdP(2)O(7)(.)0.5H(2)O has been achieved by means of isothermal thermogravimetry experiments under controlled water vapour pressure. This pressure was found to have an accelerating effect upon the rate of reaction. The kinetic curves giving the extent of reaction versus time can be described using a Mampel's model. Via this modeling, values of surfacic growth rate and frequency of nucleation have been determined. (c) 2004 Elsevier B.V. All rights reserved.