AbstractThe previous study on low mole ratio urea-formaldehyde (UF) and urea-melamine–formaldehyde (UMF) resins synthesized in the typical way as particleboard binders was repeated with the inclusion of a strong acidic reaction step at a mole ratio of 2.7 in the beginning of the resin synthesis procedure. The resulting UF and UMF resins showed longer storage- and pot-lives, longer gel times, and the particleboards gave higher internal bond and lower water-soak absorption values. However, the free formaldehyde contents of boards were increased with UF resins and decreased with UMF resins, indicating that the uron-type methylene-ether groups formed from the strong acidic step resulted in enhancement of the bonding, but they give off some extra formaldehyde, which is captured more effectively in UMF resins because of the higher reaction capacity of melamine. The extra acidic reaction step could be useful in UMF resin syntheses. The generation of extra formaldehyde by uron-type methylene-ether bonds is documented.