Reverse microemulsions, consisting of heptane, nonionic surfactants, and aqueous salt solutions, were used to synthesize BiOX (X = Cl, Br, I) nanoparticles. The reverse micelles act as nanoscale templates for the ionic precipitation process. The size of the micelles can be used to tailor the size of the particles. BiOCl nanoparticles were synthesized in a range of 3-22 nm (diameter); for BiOBr and BiOI, the range was 5.5-22 nm and 4-18 nm, respectively. The reverse microemulsions were characterized by dynamic light scattering. X-ray diffraction patterns of the particles isolated from the microemulsions clearly demonstrate the influence of the water/surfactant ratio (R_w) of the microemulsions on the crystallite size. Also, the concentration of the involved salt solutions affects the particle diameter. For BiOI nanoparticles, a significant dependence of the band gap energy on the particle diameter was observed using UV-vis spectroscopy. Depending on the size of the particles, the color varied from pale yellow to dark orange. Atomic force microscopy of BiOI nanoparticles on a mica substrate revealed well-separated particles with a narrow size distribution.