A Monte Carlo model for the coagulation of nanoparticles has been developed to predict the terminal particle sizes and particle size distribution of nanoparticles. The coagulation model is divided into two stages. In the first stage, the model includes the phenomenon of nucleation and growth via intermicellar exchange. The coagulation of nanoparticles is rapid during this stage and occurs for a very brief period after the initial mixing of reactants contained in the aqueous cores of reverse micelles. The repulsive force between particles determines the fraction of collisions which lead to a successful coagulation event. Therefore, in the second stage, the efficiency of collisions between micelles containing particles is modelled as a function of the diameters of the particles contained in them. The model predictions show a reasonable agreement with experimental data.