In this paper, we use an XRD-based 100% approach method to quantify the activity evolution of phase transformation from annealed powder Mg(OH)(2) to MgO. The sample is chemically synthesized. The isothermal phase transformation is carried out between 300 and 310 degrees C. The ratio of reacted magnesium hydroxide is applied and compared with different existing models. The Avrami model A(n) appears to be the most close-fitting. An order of reaction between 1.5 and 2 is observed that it could be considered as a complex combination of different phenomenon, for which we test different models. (C) 2009 Elsevier B.V. All rights reserved.