In this work the magnetic properties of magnetite powders doped with Co and Ni are investigated as a function of dopant concentration. Two sets of Fe3-xTxO4 powders with T=Co and Ni, x=0–1 were prepared using a hydrothermal method, with particle sizes of about 1 μm. The Mössbauer measurements revealed that both Co2+ and Ni2+ ions are located mostly on the octahedral sites, affecting the hyperfine fields and relative populations of both sites. In the case of Co-doped magnetite, the hyperfine magnetic fields increase almost linearly with increasing cobalt content. In the case of Ni-doped magnetite, the influence of annealing temperature during preparation was studied. For both subcritical and critical temperatures, the hyperfine fields of the tetrahedral and octahedral sites are larger than those corresponding to the magnetite powder. Bulk magnetic properties of these powders were studied by means of hysteresis loops recorded at 4.2 K in an applied field of 1.5 T. The results are compared with the pure magnetite powder obtained by the same hydrothermal method with similar particle sizes. It was found that Co addition significantly increases the coercive field of doped magnetite, while Ni causes its slight decrease. © 2002 American Institute of Physics.