Solid nanoparticles of nimesulide (molecule of a pharmaceutical interest) have been synthesized by direct precipitation in two water/oil (W/O) microemulsion systems: Epikuron 170 (E170, which is a lecithin)/isopropyl myristate/water/n-butanol (ME1) and E170/isopropyl myristate/water/isopropanol (ME2). The size distributions of the observed nanoparticles are relatively narrow for the two systems. In the two microemulsions, the diameter of the nanoparticles is between 45 and 60 Angstrom. The size seems to be independent of either the R factor ([water]/[E170]) or the concentration of the active compound solubilized in chloroform. The constancy of the size suggests that the size is controlled by thermodynamic stabilization of the nanoparticles with the surfactant molecules. The nature of the cosurfactant does not have an obvious influence on the nanoparticle size. The nanoparticles are instantaneously formed and remain stable for a long period of time (several months).