Several fine powder samples of ZnxMg1-xFe2O4, 0=x=1, were prepared by the supercritical sol-gel (aerogel) process, and were annealed at 500?°C for 2 h. The structure, crystallite size and inversion parameter were determined by x-ray diffraction. The crystallite diameter of the as-prepared powders ranged from 6.5 to 8.5 nm. The minimum size was achieved for the samples with 0.25=x=0.50. After annealing the crystallite size increased by an average of 3 nm. The values of the inversion parameters, before and after annealing, reflect roughly the chemical preference of Zn cations to the tetrahedral sites and Mg cations to the octahedral sites of the spinel structure. However, these values indicate that the as-prepared materials were not in the equilibrium state. The magnetic properties were studied by 57Fe Mössbauer spectroscopy over a temperature range of 25 K to room temperature. Taking advantage of their sensitivity to the superparamagnetic behavior induced by the size effects, the Mössbauer spectra were used to determine the blocking temperatures. The calculated values peaked at x=0.50 for both the as-prepared and annealed powders. SQUID magnetometry measurements are currently underway. © 1996 American Institute of Physics.