Among the most prominent challenges for the analysis of biomass pyrolysis products is the characterization of the abundant oligomer fraction of bio-oil. This fraction is principally made up of pyrolytic lignins and dehydrated, highly modified sugar oligomers, called humins, in liquid phase. An emerging technique for analysis of this oligomer fraction is high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), which allows for accurate determination of CxHyOz formulas for detected oligomers. Additionally, using simple dehydration and fragmentation reaction schemes, similar formulas can be developed from cellulose-, hemicellulose-, and lignin-derived oligomers, which are primary depolymerization products. In this study, FT-ICR MS analysis is coupled with combinatoric dehydration and fragmentation modeling to match experimentally detected bio-oil oligomers with hypothetical pathways for their formation during pyrolysis. In this way, we present a novel strategy by which oligomeric structures can be proposed for bio-oils. Using this approach, it becomes possible to advance the understanding of both the molecular structures comprising the bio-oil oligomer fraction and the pathways by which these structures form during biomass pyrolysis reactions.