Monte Carlo simulations were performed to study the formation of nanoparticles in microemulsions. The influence of the following parameters on particle size distribution (PSD) was investigated : number of reagent molecules per droplet, surfactant film flexibility, droplet size, presence or absence of autocatalysis by the product, and the fraction of bulk volume made up by droplets. It was found that autocatalyzed reactions with high reagent concentrations and low surfactant film flexibility afforded bimodal PSDs. Particle size increased with reagent concentration when this was low, but at higher reagent concentrations it depended chiefly on droplet size. The dependence of PSD on reaction time confirmed the successive occurrence of well-defined nucleation and growth processes. We have compared the simulation results with experimental data taken from different authors and carried out in our laboratory. Good agreement between both kind of results supports the conclusions of this paper.