Enzymatic oxidation of two phenolic compounds [syringic acid (3,5-dimethoxy-4-hydroxybenzoic acid) and 2,6-dimethylphenol] was studied. The products of laccase- and laccase-mediator-catalyzed oxidation reactions were monitored by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and further analyzed by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) MS with collision-induced dissociation (CID) experiments. For the oligomers of syringic acid, some variability was observed in MALDI-TOF analysis. However, the origin of this variability could not be resolved on the basis of MALDI-TOF spectra due to the poor resolution of the instrument in use. The strength of ESI-FTICR MS was the high-resolution data provided from oligomers of syringic acid. The CID experiments were extremely useful for structural studies of oligomers and verified that the variability of the products was due to the end groups; the phenolic hydroxyl group was modified during the oxidation.