An oil-water emulsifier comprises a Venturi member having an inlet for receiving oil, an oil-water emulsion outlet and an opening extending therethrough from the inlet to the outlet. The opening of the Venturi member comprises a diameter-reducing portion which connects to a throat portion having a substantially smaller diameter than the inlet, the throat portion being connected to an expanding portion extending from the throat to the outlet, the diameter of the outlet of the opening being substantially greater than that of the throat portion. A plurality of water injection holes extend from the outer periphery of the Venturi member to the throat portion so as to be in communication with the oil flowing through the throat portion, the injection holes being preferably substantially perpendicular to the direction of oil flow through the throat portion. Also disclosed is an oil-burner boiler system incorporating the above-described oil-water emulsifier.