Industrial Tokyo and Sinter 75 nickel oxides were reduced with 35 and 80% volume hydrogen-argon mixtures between 350 and 1000 degrees C in a TGA apparatus. Several abnormalities were observed. At low temperature and in 35% H-2, an incubation period was observed. As the temperature was increased the rate of reduction increased up to some temperature, following which a decrease in the reaction rate was observed, which again was followed by an increase in the rates as the temperature exceeded 900 degrees C. It was found that reduction kinetics of these oxides strongly depend on the morphological features of the oxides. It became clear that the shrinking core model was not applicable and instead the kinetic parameters were assessed by the grain model. (C) 2009 Elsevier Ltd. All rights reserved.