Tannin-formaldehyde (TF) thermosetting wood adhesives for panel products for the building industry were examined to determine which oligomers were formed during the reaction between tannin and formaldehyde. The reaction products were examined by Matrix Assisted Laser Desorption/Ionization Time of Flight mass spectrometry. The analysis revealed that at the gel point, the adhesive was composed of unreacted flavonoid monomers and some oligomers, and due to reaction with formaldehyde, of methylolated tannin monomers and oligomers as well as flavonoids linked by methylene bridges. The slow gelling conditions used to prepare the TF hydrogel caused the unusual occurrence that not only the flavonoid units A-rings but also pyrogallol-type B-rings of the flavonoid units started to react with formaldehyde at the low pHs between 6 and 7 used for tannin adhesives and contributed to the network formed. This reaction was observed for the first time in this study.