A new method for evaluation of isothermal conversion curves from the experimentally determined nonisothermal conversion curves for the nickel oxide reduction process in hydrogen atmosphere was established. It was concluded that by applying the conventional and proposed prediction methods it was not possible (at T-iso <= 300 degrees C) to calculate the isothermal conversion curves using the experimentally obtained nonisothermal conversion curves for this reduction process. The dependence of the apparent activation energy on the degree of conversion shows that the investigated reduction process is complex under isothermal or nonisothermal conditions. By applying Miura's procedure, the shape of density distribution functions of the apparent activation energies was determined for the isothermal and nonisothermal reduction processes. It was concluded that the existence of different distributions of internal energy of the NiO reduction centers is a consequence of different reduction kinetics under isothermal and nonisothermal experimental conditions.