Abstract: An apparatus and process for preparing a water/diesel oil micro-emulsion including a diesel oil feeding unit, an emulsifying composition feeding unit, a water feeding unit, and a mixing tank in fluid communication therewith. A duct extends along main direction (X-X) and presents an inlet opening, outlet nozzle, and a cone shaped septum coaxial with respect to the main direction (X-X) and tapers towards the outlet nozzle. A plurality of holes is in the conical wall of the septum and a plurality of lamellae is arranged in the duct downstream of the septum. The plurality of lamellae creates a plurality of small chambers. Holes and the small chambers delimit a labyrinth passageway for liquid flowing through the duct towards the outlet nozzle. A batch is recirculated through the mixing tank and a conduit including a mixing device and then the fuel micro-emulsion batch is discharged.