The mechanisms involved in the formation of high solids content composite polymer-monomer waterborne miniemulsions in a high-pressure homogenizer were investigated combining experimental results and a mathematical model for the process. It was found that the final droplet size was the result of two consecutive processes: droplet break-up and coagulation. The final droplet size was determined by the mechanism giving the largest droplet size (C) 2007 American Institute of Chemical Engineers.