The gelling process of urea-formaldehyde resins has been investigated by rheological methods. The gel point values were determined by three methods, and compared with those obtained by low resolution impulsional H-1-nuclear magnetic resonance. The activation energy of the curing process was determined and the effect of the molar ratio was investigated. In addition, the critical power exponents were calculated and the parameters of the relaxation modulus function were also determined. Equations describing the gel time as a function of degree of conversion and reactants molar ratio are presented. (C) 2000 John Wiley & Sons, Inc.