The thermal decomposition behavior of magnesium hydroxide (Mg(OH)(2)) nanopowders was investigated by positron annihilation lifetime spectroscopy (PALS), X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC) analysis, high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). It was indicated that the microstructural changes started from 300 degrees C were earlier than the phase transformation from hexagonal Mg(OH)(2) nanopowders to face-centered cubic MgO occurred at about 380 degrees C during decomposition process. The variation of positron annihilation parameters revealed the production of new vacancy defects and the aggregation of vacancy clusters in grain boundary areas and microvoids between particles due to the removal of H2O and rearrangement of interface atoms. A positron trapping model was proposed to help further understand the changes of microstructure and interfacial defects during Mg(OH)(2) thermal decomposition. (C) 2017 Elsevier B.V. All rights reserved.