A method of entraining a second fluid in a first fluid is provided. The method comprises supplying a first fluid to a processing passage (4) having an inlet (6) and an outlet (8), and supplying an entrainment fluid to a nozzle (10) which opens into the processing passage (4) intermediate the passage inlet (6) and the passage outlet (8). A second fluid which will undergo a change of phase and/or state when added to the first fluid is also provided, and supplied to a first port (22) opening into the processing passage (4) adjacent the nozzle (10). The entrainment fluid is injected from the nozzle (10) into the processing passage (4) so as to form a dispersed phase of the first and second fluids in a continuous vapor phase, and the vapor phase is condensed downstream of the nozzle (10). A device suitable for carrying out such a method is also provided.