Primary pyrolysis reactions and relative reactivities for depolymerization and condensation/carbonization were evaluated for various lignin model dimers with alpha-O-4, beta-O-4, beta-1, and biphenyl substructures by characterizing the tetrahydrofuran (THF)-soluble and THF-insoluble fractions obtained after pyrolysis at 400 degrees C. Reactivity was quite different depending on the model structure: depolymerization: alpha-O-4 [phenolic (ph), nonphenolic (nonph)], beta-O-4 (ph) > beta-O-4 (nonph), beta-1 (ph, nonph) > biphenyl (ph, nonph); condensation/carbonization: beta-1 (ph) > beta-O-4 (ph) > alpha-O-4 (ph) > beta-O-4 (nonph), biphenyl (ph, nonph), alpha-O-4 (nonph), beta-1 (nonph). Major degradation pathways were also identified for beta-O-4 and beta-1 model dimers: beta-O-4 types: C-beta-O cleavage to form cinnamyl alcohols and phenols and C-gamma-elimination yielding vinyl ethers; beta-1 types: C-alpha-C-beta cleavage yielding benzaldehydes and styrenes and C-gamma-elimination yielding stilbenes. Relative reactivities of these pathways were also quite different between phenolic and nonphenolic forms even in the same types; C-beta-O cleavage (beta-O-4) and C-gamma-elimination (beta-1) were substantially enhanced in phenolic forms.