The paper gives results of tests of two commercially available water-in-oil emulsifiers to determine the effect of water/residual oil emulsions on air pollutant emissions and thermal efficiency of a packaged boiler. Of the two emulsifiers, one (Cottell reactor) used low pressure mechanical and ultrasonic energy, and the other (Total emulsifier) used high pressure mechanical energy. Each emulsifier was tested at a constant oil flow rate with varying amounts of water over a range of stoichiometric ratios. Water/residual oil emulsions from both emulsifiers significantly reduced particulate mass. With the Cottell reactor, smoke emissions were significantly reduced, whereas, with the Total emulsifier, smoke emissions increased and peaked with a 20 percent water emulsion, then decreased as water was increased to 30 percent. Neither emulsifier had any significant effect on emissions of NO, SO2, or HC when firing water/residual oil emulsions. Both emulsifiers were capable of improving efficiency by approximately 2 percent under certain conditions.