Enormous analytical challenges are involved in the analysis of compositionally complex samples such as biomass-derived products and crude oils. In this dissertation mass spectrometry (MS)-based novel strategies for the characterization and identification of chemical components in pyrolysis bio-oils are presented. Several techniques including, accurate mass analysis (e.g., Kendrick mass defect analyses, chemical formula determinations, and van Krevelen plots), collision-induced dissociation (CID) and mass – mobility relationships are utilized to support molecular-level investigations. 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-mode electrospray ionization-Orbitrap and ion mobility-time-of-flight (IM-TOF) MS. Oxygenrich species with a high degree of unsaturation were observed, indicating that catalytic upgrading will likely be required if slow-pyrolysis bio-oils are to be utilized as fuel. Additionally, results from analyses of the PS oily fraction subjected to a sequential solvent fractionation demonstrated a partial separation and enrichment of compounds according to oxygen classes, where oxygen classes generally trended with solvent polarities. Generally, higher oxygen classes were preferentially enriched in higher polarity solvents. Mass – mobility correlations were investigated by IM-MS for various structurallyunique homologous series composed of commercially-available compounds. Structural variation involved the inclusion of different repeat units in oligomeric series and different terminal groups in CH2-homologous series. Mass – mobility correlations were also investigated for select CH2-homologous series identified in a bio-oil and compared with results observed for commercial series. A linear mass – mobility correlation (R 2 ≥ 0.996) was established for all series except those in which a substantial change in the gas-phase conformation of ions was probable. Slopes observed for CH2-homologous series with a single terminal group were significantly steeper than slopes observed for series containing two terminal groups. Additionally, a correlation between slope and double bond equivalents suggested that the CH2-homologous series identified in the bio-oil were structurally similar to commercial series containing two terminal groups. Additionally, IM-MS and IM-MS/MS analysis of ions belonging to select CH2-homologous series suggested 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.