Several emulsions consisting of biomass pyrolysis oil (bio-oil) in diesel fuel were produced and analyzed for stability over time. An ultrasonic probe was used to generate microscopic droplets of bio-oil suspended in diesel fuel, and this emulsion was stabilized using surfactant chemicals. The most stable emulsion was produced using a polyethylene glycol dipolyhydroxystearate (PEG-DPHS) surfactant, with a hydrophilic-lipophilic balance (HLB) number of 4.75 and a 32:8:1 ratio of diesel to bio-oil to surfactant, i.e., 20% utilization of bio-oil. This emulsion consisted of uniformly sized droplets with an average diameter of 0.48 mu m, with no observed coalescence of droplets after 1 week. If left undisturbed, these droplets would slowly settle to the bottom of the mixture at a rate of only 2.4 mm/day, but this settling can be eliminated with slight mechanical agitation. This level of stability facilitates utilization of 20 wt 96 raw bio-oil in diesel as a renewable liquid fuel for spray combustion without the need for costly and energy-intensive upgrading. Additionally, GC/MS analysis was used to investigate the relative concentrations of various bio-oil components in the emulsions. This analysis identified levoglucosan as a bio-oil component that may be responsible for the instability of the emulsions. Experiments with bio-oil produced by catalytic fast pyrolysis over HZSM-5 (CFP oil) revealed that the major components of this oil are directly miscible with diesel fuel without the need for an emulsion.