A particleboard adhesive-type urea-formaldehyde (UF) resin was made at a formaldehyde ratio of 2.10 and added with a second urea at low temperature to the typical final formaldehyde/urea ratio of 1.15. Time samples taken during heat treatments of the resin sample up to 70 degrees C over a period of 250 min showed decreases in Type II/IIi hydroxymethyl group content, accompanied with decreases in resin sample viscosity and increases in formaldehyde emission of bonded particleboards. The results indicate that various hydroxymethyl groups of polymeric UF resin components migrate to the second urea to form Type I hydroxymethyl groups. Time samples taken during the room-temperature storage of the resin sample over a period of 1 month behaved similarly initially, but in the later stage, some polymerization progressed, shown by increases in viscosity and methylene and methylene-ether group contents. (C) 2000 John Wiley & Sons, Inc.