The structure-reactivity relationship in fast pyrolysis of lignin for monomeric phenolic compounds was studied on seven lignins from hardwood, softwood, and grass. The distribution of elements, functional groups, phenylpropane units, and interunit linkage bonds varied greatly across the lignins. Lignins from hardwood and grass prepared with mild conditions presented more C-O linkages. Pyrolysis-gas chromatography/mass spectrometry showed that the cleavage of unstable C-O linkages dominated the pyrolysis reaction at low temperatures, and the total yields of monomers peaked at 700 degrees C for most lignins. 4-Vinylphenol produced from acid extracted corn stalk lignin reached up to 4.77 wt % at 700 degrees C. Pyrolysis of grass lignins and woody lignins, which had more unstable C-O linkages, exhibited higher total yields of monomers. The pyrolysis behavior of lignin at low temperatures is closely related to its structural characteristics. Pyrolyzed at 500 degrees C, the total yields of monomeric aromatics were highly linearly correlated with the frequencies of C-O linkages (R-2 = 0.86). This work demonstrates the significance of selecting the right lignin for producing monomeric aromatic compounds.