Nanocrystalline, superparamagnetic MnZn-ferrite particles with a narrow size distribution were prepared using precipitation in a reverse microemulsion system consisting of hexanol as an oil phase and CTAB as surfactant and aqueous solutions of Mn2+, Zn2+ and Fe2+ Sulfates. Tetramethyl ammonium hydroxide served as a precipitating agent and hydrogen peroxide as ail oxidizing reagent. The influence of the processing parameters - the pH value after precipitation of the hydroxides, the temperature and the time of the reaction - on the nature of the product nanoparticles was studied. It was found that the pH value after precipitation should be kept above 8 to obtain a pure spinel ferrite product. The particle size increased with increasing pH and temperature of the reaction, whereas the aging time after precipitation of the hydroxides had only a minor effect on the nature of the nanoparticles.