In this research, two different types of commercial tannins, namely a hydrolysable tannin (chestnut) and a condensed flavonoid tannin (mimosa), were used to prepare two types of soy-based (soy flour (SF) and soy protein isolate) adhesives for making plywood. Thermogravimetric properties (TGA) and its derivative as function of temperature (DTG) of different soy-based adhesive were measured in the range 40 degrees C-300 degrees C. Thermomechanical analysis (TMA) from 25 degrees C to 250 degrees C was done for the different resin formulations. Duplicate three-ply laboratory plywood panels were prepared by adding 300 g/m(2) of the adhesives' total resin solid content composed of SF or isolated soy protein (ISP), urea, chestnut, and mimosa tannin extracts with hexamine as hardener. Based on the results obtained, tannins can improve SF adhesion properties. The TMA showed that chestnut tannin extract appeared to react well with SF, while mimosa tannin extract appeared to react well with ISP. Matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry also showed that among other reactions, the soy protein amino acids reacted with the tannins. Furthermore, delamination and shear strength test results showed the good water resistance of plywood bonded with soy-based tannin modified adhesive.