Co, Ni, Mn, Cr, and Cu substituted magnetites were prepared by the hydrothermal method at 300 °C, with concentrations ranging from 8.2&percent; to 12.5&percent;. Transmission electron microscopy determined the average particle diameter ⟨&Fgr;⟩ to be in the hundred of nm range and the morphological modifications induced by the various substitutions employed. Hysteresis loop measurements were performed to determine the coercive fieldHc,and saturation magnetic momentms.WhileHcdecreased with increasing ⟨&Fgr;⟩, the particle shape was found to play an important role in explaining the dependence ofmson ⟨&Fgr;⟩. Transmission Mo¨ssbauer spectroscopy was used to determine the site preference of the substitutions and their effect on the hyperfine magnetic fields. The room-temperature Mo¨ssbauer spectra were analyzed assuming a random distribution of substitutents using the binomial distribution from the ionic crystal point of view. Superparamagnetic particles were observed at room temperature in the case of Cu and Cr substituted magnetites. ©1998 American Institute of Physics.