Nickel oxide (NiO) granules formed by vapour deposition from a chloride solution, were reduced in hydrogen using thermal gravimetry. In the temperature range from 400 to 600 degrees C, the rate of reduction increased with increasing temperature and increasing hydrogen pressure. Microscopic analysis showed that in this temperature range the reaction followed the shrinking core model. The activation energy for reduction was found to be 90kJ/moI in this temperature range. When the temperature was increased above 600 degrees C, the reduction rate decreased noticeably before suddenly becoming extremely fast above 950 degrees C. Microscopy showed inner bands of metallization inside the outer metallic rim, indicating partial blockage of the interface between the reacted and un-reacted NiO core. At these temperatures, particle agglomeration and sintering were also found to take place. Surface segregation of sulphur may account for the slow-down in reduction rate above 600 degrees C. (c) 2005 Elsevier Ltd. All rights reserved.