This paper describes the distribution of Fe and Ni between the octahedral and tetrahedral sites in pentlandite (Fe,Ni)9S8. The dependence of the distribution on pressure and temperature and the activation energy of the cation exchange reaction were determined through annealing experiments. Synthetic crystals were annealed at 433-723 K and pressures up to 4 GPa, and natural crystals were annealed at 423, 448 and 473 K in evacuated silica capillary tubes for various durations. The cation distributions in the synthetic crystals were determined with an X-ray powder method employing the anomalous dispersion effect of CuKc~ and FeK~ radiations, while those of natural crystals were calculated from the cell dimensions. The values of A U, AS and A Vfor the Fe/Ni exchange reaction are -6818 J tool -~, 20.52 J K -1 mo1-1, and 6.99 x 10 -6 m a tool -1, respectively. The dependence of the Fe/Ni distribution on pressure (Pa) and temperature (Kelvin) was determined as inK =2.47+8.20x102T-~+8.41x10 -7T -1P, where K = (Fe/Ni)oct,hedral/(Fe/Ni)tetrahedral. The activation energy of the cation exchange reaction was 185 kJ mol-1.