A thorough knowledge of liquid fuel properties is required for optimizing the design of spray combustion systems. To this effect, the density, theological characteristics, and surface tension of a hardwood-derived bio-oil fraction were measured between 25 and 80 degrees C. Particular attention was paid to how these properties would govern the potential spray behavior of the fuel. Whenever possible, results were compared to No. 6 residual or heavy fuel oil, which is the closest petroleum counterpart. Steady and dynamic rheological tests were performed that indicate that the bio-oil is mildly shear thinning, weakly elastic, and predominantly Newtonian in nature. Direct visualization of hanging pendant drops allowed for the measurement of non-equilibrium and equilibrium surface tension. Small differences were found between these values, and the dependence of surface tension on the temperature was weak. Details about the microstructure of the fuel were examined using optical microscopy. Although the bio-oil demonstrates macroscopic homogeneity and phase stability, it is microscopically composed of a tar-like, char-laden phase immersed within a bulk aqueous phase. The atomization quality of the liquid fuel was evaluated using the Ohnesorge number. On the basis of its liquid properties, the bio-oil is expected to produce a spray that is comparable to heavy fuel oil. Heating value, water content, pH, and elemental composition were also measured and fell within the expected range for bio-oils. Special considerations and the adaptation of standard experimental techniques to accommodate working with the fuel have been emphasized to facilitate future studies.