The spinel ferrite system NiMn x Fe 2 - x O 4 , with x=0, 0.2, 0.4, 0.6, 0.8 and 1, was prepared by the standard double sintering ceramic method. The cation distribution has been calculated analytically for the first time using X-ray diffraction combined with computer technology. The results show, with increasing manganese ion substitution, that both Ni 2 + and Mn 3 + ions predominately occupy the octahedral sites. This is consistent with their preference for large octahedral site energy. The concentration of Mn 3 + ions in octahedral sites increases while that of Fe 3 + ions decreases linearly.