AbstractStudies on the softening behavior of in situ lignin of normal wood in a given species have never been performed before due to the relatively narrow lignin content and lignin structural variation within one species. Using transgenic trees with different levels of lignin content and/or syringyl to guaiacyl propane (S/G) ratio helped us to overcome this problem. Submersion three-point bending and parallel-plate compression-torsion dynamic mechanical analyses were conducted on one-year-old wild type and transgenic aspen (Populus tremuloides Michx.). The different genetic modifications included groups with reduced lignin content, increased S/G ratio, and both reduced lignin content and increased S/G ratio. Measurements with both methods revealed a statistically significant decrease in glass transition temperature in the reduced-lignin genetic group compared to the wild-type. Increase in the S/G ratio did not affect the thermo-mechanical properties; these results contradict claims that increasing the methoxyl groups would reduce lignin cross-linking and the glass transition temperature.