NiO/gamma-Al2O3 catalysts with NiO content of 9 wt% and 24 wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100 degrees C to 1000 degrees C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime tau(3) and tau(4), which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime tau(4) is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both tau(4) and its intensity I-4, which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600 degrees C, abnormal increase of the lifetime tau(4) is observed. This is due to the formation of NiAl2O4 spinel from the reaction of NiO and gamma-Al2O3. The generated NiAl2O4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime tau(4). Formation of NiAl2O4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements. (C) 2012 Elsevier B.V. All rights reserved.