A novel device is applied to produce emulsions of methyl methacrylate in water with a controllable size in the range of 30100?nm. The process is based on the reciprocating flow of the material through an abrupt contraction which generates a strong elongational flow. This results in highly efficient dispersive mixing even at moderate pressures, thus reducing viscous dissipation and improving temperature control. The original design of the device also allows easy feeding and sampling, easy adjusting of the total volume of the emulsion, and processing volatile components owing to the liquid- and gas-tightness of the device. The influence of process parameters (like pressure drop and mixing time) and composition of the system (volume ratio of the dispersed and continuous phases, surfactant and hydrophobic agent weight percentages) on the droplet size and stability of the emulsions is investigated and discussed.