Nanocrystalline particles of magnesioferrite (MgFe2O4) were prepared by a sol-gel/combustion method using iron nitrate, Fe(NO3)3.9H2O, magnesium nitrate, Mg(NO3)2.6H2O, and citric acid, C6H8O7.H2 O, and annealed for 2 hours at 400, 500 and 600 °C. The average particle size, determined by X ray diffraction, was found to depend on the annealing temperature and varied from ⟨D⟩ = 8.1 to ⟨D⟩ = 17.8 nm. By measuring at several temperatures the relative intensity of the Mössbauer spectra due to superparamagnetic particles and to ferrimagnetic particles, we determined the size distribution of the nanoparticles in the samples. It was found to be a log-normal distribution with a most probable diameter that varied from Dm = 6.4 to 17.2 nm and a full width at half-height ΔD in the 5-6 nm range.