In an investigation of the combustion behavior of biomass-derived liquids, we have performed single droplet experiments with two biomass oils, produced from the pyrolysis of oak and pine. The experiments are conducted at 1600 K on 320 mu m diameter droplets introduced into a laminar flow reactor, operating at O-2 concentrations of 14-33 mol %. In-situ video imaging of burning droplets reveals that biomass oil droplets undergo several distinct stages of combustion. Initially biomass oil droplets burn quiescently in a blue flame. The broad range of component volatilities and inefficient mass transfer within the viscous biomass oils bring about an abrupt termination of the quiescent stage, however, causing rapid droplet swelling and distortion, followed by a microexplosion. Droplet coalescence follows, and subsequent burning occurs in a faint blue flame with occasional smaller scale bursts of fuel vapor. At the late stages of biomass oil combustion, droplets are accompanied by clouds of soot, produced from gas-phase pyrolysis. Liquid-phase polymerization or pyrolysis of the oxygenate-rich biomass oils leads to the formation of carbonaceous cenospheres, whose burnout signifies the final stage of biomass oil droplet combustion. Oak and pine oils behave similarly during combustion, though differences in their physical properties cause pine oil to show more susceptibility to fragmentation during the microexplosion. Changes in oxygen concentration alter the timing of the events during biomass oil combustion, but not their nature. Comparison of the biomass oils with No, 2 fuel oil reveals vast differences in combustion mechanisms, which are attributable to differences in the physical properties and chemical compositions of the fuels. Despite these differences, the biomass oils and No. 2 fuel oil exhibit surprisingly comparable burning times under the conditions of our experiments.