An experiment was conducted to investigate the effects of the resin solid content, catalyst content, and pH value obtained by the addition of two kinds of catalysts on the gel time of a urea-formaldehyde (UF) resin. Upon the addition of ammonium chloride, the pH value of the resin mixture decreased to 7 but not significantly further because of the limited free formaldehyde in the system. The pH values of the critical points, at which the resin-curing rate dramatically increased and the gel time was reduced, were above 7 for both catalysts. To achieve the same gel time, the required pH value of the OF resin adjusted with ammonium chloride was higher than that of the resin modified by hydrochloric acid. This indicated that the main effects of ammonium chloride on the UF-resin cure included both the release of hydrochloric acid and the catalysis of the reactants in the UF-resin system. The gel time of the OF resin obviously decreased with increasing catalyst and resin solid contents and with decreasing pH. (c) 2006 Wiley Periodicals, Inc.