Reverse microemulsions were used to synthesize silica nanopaticles. Phase behavior experiments were performed to characterize one phase water-in-oil microemulsion region in ternary systems containing water(NH3), surfactant, and oil. Of particular interest was the effect of cosurfactant and oil on formation of one phase microemulsion where five different types of surfactants and four different oils such as n-decane, n-heptane, cyclohexane, and isooctane were used. Both surfactant and oil caused a substantial change in microemulsion phase behavior and thus SiO2 nanoparticle characteristics made from one phase water-in-oil microemulsion. The use of a cosurfactant in reaction conditions provided control over particle size; smaller particles resulted by limiting the aqueous volume while simultaneously increasing the amount of cosurfactant for a given concentraiton of reactants. Photographs from transmission electron microscopy showed that average particle size, particle size distribution and number of particles formed were depend on microemulsion composition, especially water contents solubilized in the microemulsion region.