In this paper we have studied by Monte Carlo simulation the influence of reactant excess (x) and film flexibility (f) on the final size of particles obtained by reaction in microemulsions.;We have observed that the ripening mechanism is more significant as reactant excess and film flexibility increase because, when one of the reactants is exhausted, ripening is the only possible way of growth. This gives rise to a faster growth process. Experimental results of particle sizes for different x values agree with the results obtained by simulation, which provides an explanation of the growth mechanism. These results show that both parameters (x and f) are important in order to explain the final size distributions of particles obtained in microemulsions.