Pyrolysis of solid biomass, in this case pine pellets and peanut hulls, generates a hydrocarbon-rich liquid product (bio-oil) consisting of oily and aqueous phases. Here, each phase is characterized by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to yield unique elemental compositions for thousands of compounds. Bio-oils are dominated by O-x species: few oxygens per molecule for the oily phase and many more oxygens per molecules for the aqueous phase. Thus, the increased oxygen content per molecule accounts for its water solubility. Peanut hull bio-oil is much more compositionally complex and contains more nitrogen-containing compounds than pine pellet bio-oil. Bulk C, H, N, O, and S measurements confirm the increased levels of nitrogen-containing species identified in the peanut hull pyrolysis oil by FT-ICR MS. The ability of FT-ICR MS to identify and assign unique elemental compositions to compositionally complex bio-oils based on ultrahigh mass resolution and mass accuracy is demonstrated.