Water-resistant bonds are important in many wood products and have been hard to obtain with many bio-based adhesives. Using a three-step process, water-soluble soy flour has been converted into adhesives that cure into an insoluble material for water-durable adhesives. The process consists of denaturation of soy flour, followed by modification with formaldehyde and then conversion via copolymerization with a suitable cross-linking agent into an insoluble material. Both formaldehyde and phenol-formaldehyde are used as cross-linking agents. The extent of conversion into the cross-linked copolymer was assessed with a 24-h water extraction procedure and via elemental analysis. Soy-based resins with 44-86% conversion of soluble soy flour have been successfully prepared with up to complete conversion of the protein component. These resins were also used to prepare strandboards of comparable performance to a control commercial phenol-formaldehyde resin. A direct relationship between the percentage of soy flour incorporated and the final board performance was obtained.