The effects of calcination temperature and doping with K2O on solid-solid interactions and physicochemical properties of NiO/Fe2O3 system were investigated using TG. DTA and XRD techniques. The amounts of potassium, expressed as mol% K2O were 0.62, 1.23, 2.44 and 4.26. The pure and variously doped mixed solids were thermally treated at 300, 500, 750, 900 and 1000 degrees C. The catalytic activity was determined for each solid in H2O2 decomposition reaction at 30-50 degrees C. The results obtained showed that the doping process much affected the degree of crystallinity of both NiO and Fe2O3 phases detected for all solids calcined at 300 and 500 degrees C. Fe2O3 interacted readily with NiO at temperature starting from 700 degrees C producing crystalline NiFe2O4 phase. The degree of reaction propagation increased with increasing calcination temperature. The completion of this reaction required a prolonged heating at temperature >900 degrees C. K2O-doping stimulates the ferrite formation to an extent proportional to its amount added. The stimulation effect of potassium was evidenced by following up the change in the peak height of certain diffraction lines characteristic NiO, Fe2O3, NiFe2O4 phases located at "d" spacing 2.08, 2.69 and 2.95 angstrom, respectively. The change of peak height of the diffraction lines at 2.95 angstrom as a function of firing temperature of pure and doped mixed solids enabled the calculation of the activation energy (DE) of the ferrite formation. The computed Delta E values were 120. 80, 49, 36 and 25 kJ mol(-1) for pure and variously doped solids, respectively. The decreasein DE value of NiFe2O4 formation as a function of dopant added was not only attributed to an effective increase in the mobility of reacting cations but also to the formation of potassium ferrite. The calcination temperature and doping with K2O much affected the catalytic activity of the system under investigation. (c) 2008 Elsevier B.V. All rights reserved.