In a process for producing stable emulsions of at least two substantially immiscible fluids, particularly emulsions of a liquid fuel with water, a considerable increase in the efficiency of the emulsion-formation process is achieved by injecting the fluids to be emulsified into an emulsification chamber provided with an injection system which imparts to the fluids a motion in a direction which is substantially perpendicular to the general direction in which the fluids travel through the emulsification chamber. This injection system can be provided by means of a diffuser having an inlet hole, through which a stream of liquid is fed in a substantially axial direction, and at least one outlet hole, which leads into the chamber and whose axis lies on a plane which is substantially perpendicular to the direction of the inlet stream. In this manner, the stream strikes the walls of the emulsification chamber, producing a turbulent fluid motion which has a predominantly helical orientation and is capable of producing an efficient dispersion of one fluid in the other, forming dispersed-phase particles having an average diameter on the order of one micron or even less.