Bio-oils produced from biomass pyrolysis are an attractive fuel source that requires significant upgrading. Before upgrade strategies can be developed, the molecular composition of bio-oils needs to be better understood. In this work, oily and aqueous fractions of bio-oils produced by slow pyrolysis of two feedstocks, pine shavings (PS) and corn stover (CS), were analyzed by negative electrospray ionization (ESI)-Orbitrap and ion mobility-time-of-flight mass spectrometry (IM-TOF-MS). Analyte ion signal was observed primarily between m/z 80 and 450 in the mass spectra of these samples. Mass defect analysis and collision-induced dissociation (CID) experiments performed on mobility-separated ions indicated a high degree of homology among bio-oil samples produced from both feedstocks. Oxygen-rich species having between 1 and 9 oxygen atoms and with double bond equivalents (DBEs) ranging from 1 to 15 were identified, indicating that catalytic upgrading will likely be required if slow-pyrolysis bio-oils are to be utilized as fuel. IM-MS and IM-MS/MS analysis of ions belonging to select CH2-homologous series suggest that mass-mobility correlations and post-ion mobility CID mass spectra may be useful in defining structural relationships among members of a given Kendrick mass defect series.