Using a water-in-oil microemulsion system, silica nanoparticles containing superparamagnetic iron oxide (SPIO) crystals have been prepared and characterized. With this method, the loading of iron oxide crystals, the thickness of the silica shells, and the overall particle sizes are tunable. Moving from low to high water concentration, within the microemulsion region, resulted in a gradual shift from larger particles, ca. 100 nm and fully loaded with SPIOs, to smaller particles, ca. 30 ran containing only one or a few SPIOs. By varying the amount of silica precursor, the thickness of the silica shell was altered. Field dependent magnetization measurements showed the magnetic properties of the SPIOs were preserved after the synthesis.