Phenol-urea-formaldehyde (PUF) resins of high urea content were prepared at different hydroxyl/phenol (OH/P) mole ratios and formaldehyde/(phenol+urea) [F/(P+U)] mole ratios. The effect of synthesis parameters including OH/P and F/(P+U) mole ratios on the structure, composition, and curing characteristics of PUF resins were investigated by using both liquid (13)C nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The NMR analysis indicated that an increase in the OH/P mole ratio and/or F/(P+U) mole ratio decreased the amount of unreacted urea and monosubstituted urea, and promoted the formation of polysubstituted urea. The DSC results showed that the higher OH/P mole ratio and/or F/(P+U) mole ratio of PUF resins resulted in a lower curing temperature. The F/(P+U) mole ratio of PUF resins seemed to have a more significant accelerating effect on the curing reaction than the OH/P mole ratio.