The paper presents the performance and air pollutant emissions of an experimental water/oil emulsion burner. The burner was fired with two residual oils at selected emulsion water fractions. In addition, various stoichiometric ratios and two load conditions were used to determine how operational parameters influenced the results. Particulate mass emissions and particle size distributions are presented. Examples show that, even though particulate mass may decrease, the total amount of fine particulate emissions may increase by using water/oil emulsions. The performance of the boiler is reduced when large fractions of water are used in the emulsion due to latent heat losses. This performance loss may be only slightly recovered by operating at a reduced stoichiometric ration corresponding to a smoke limit. NOx and CO emissions data are also presented for various test conditions, but neither were affected significantly by use of water/oil emulsions.