AbstractDefatted soy-based flour (DSF) modified with a combination of acid, salt, dicyandiamide, and alkali for preparing soy-based bio-adhesives (SBA) was investigated in this study. The resulting modified products from different reaction stages were characterized by FTIR, XPS, and TG. The results from FTIR and XPS indicated that the hydrolysis of the amide link and decarboxylation had occurred after the reaction by acid and salt; these reactions resulted in an increase of active groups, such as primary amine, carboxyl, and hydroxyl groups. The active groups were further increased by treatment with dicyandiamide and alkali. Curing the SBA resulted in the condensation and cross-linkage between active functional groups. Moreover, TG analysis proved that the active functional groups were increased during the modification process of the DSF, which was consistent with the results presented in FTIR and XPS. Finally, SBA was applied to plywood made from four wood species (basswood, Pinus massoniana, Triplochiton scleroxylon, and poplar) to test its water resistance performance. The average bonding strength between wood species was close to 0.91 MPa, which demonstrated that the SBA can be regarded as an alternative to petro-based adhesives.