Crystalline Fe3O4 nanoparticles were prepared via chemical precipitation in reverse micellar cavities in a microemulsion, followed by in situ thermal annealing of the microemulsion at 150 degrees C. A valuable feature of the annealing is that the physical properties of the nanoparticles are preserved in the crystallization process, and the resulting crystalline Fe3O4 nanoparticles remain in stable suspensions. The method thus allows the control of average sizes in the final crystalline nanoparticles by varying the initial microemulsion parameters. Results from the characterization of the nanoparticle samples are presented and discussed.