Lanthanide fluoride nanoparticles of varying crystallinity, shape and size can be prepared by precipitation in reverse microemulsions of water in cyclohexane, stabilized with polyoxyethylene isooctylphenyl ether. Monodisperse particles, ranging in size from 10 to 350 nm are obtained, with size control being achieved through the variation of the surfactant to water ratio. Although the initial precipitation is rapid, dynamic light scattering measurements and TEM observations indicate that particle growth continues over a period of about 20 hours. Importantly, the final particle size far exceeds the dimensions of the micelles. Ongoing experiments, designed to shed light on the mechanism of size control, include the systematic variation of concentration as well as the determination of the number ratio of particles to micelles.