Phenol-urea-formaldehyde (PUF) resole resins were synthesized and analyzed by both liquid and solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy. The liquid C-13 NMR analysis indicated that the co-condensation reactions between the phenolic ring and the urea unit occurred during the synthesis of the resins. The addition of the urea component effectively reduced the free formaldehyde content in the resin systems. Methylene ether bridges in the resins were found to be mainly associated with the urea units. pH had significant influences on the structure and composition of the resins. Solid-state C-13 NMR measurements of the cured resins suggested that the pH probably affected the curing mechanism. A longer time and a higher temperature can generally accelerate the curing process and increase the rigidity of the cured network. (C) 2004 Elsevier Ltd. All rights reserved.