Urea-formaldehyde resins modified by melamine were synthesized by four catalysts (H2SO4, HCl, H3PO4, and NaOH/NH4OH) with a F/U/M molar ratio of 1.38/1/0.074. Resin structure and thermal behavior were studied by C-13-NMR and DSC techniques. For H2SO4, HCl, and H3PO4 catalysts, resins were prepared by two stage pH adjustment: the first pH stage was set at 1.25 (H3PO4 pH 1.60) and second pH stage was set at 5.0. For the NaOH/NH4OH catalyst, the resin was set at pH 5.0 from the start. Of the four catalysts, HCl catalyzed resins, with the highest free urea and lowest free formaldehyde, consistently yielded the lowest formaldehyde emission; NaOH/NH4OH catalyst resulted in the best IB strength tested at dry conditions and also after 24 h cold water soak and the lowest water absorption and thickness swell. The resins catalyzed with H3PO4 had the highest free formaldehyde and no free urea yielding the highest formaldehyde emission. Each DSC thermogram was proceeded by a weak exothermic peak and followed by an obvious endothermic peak. The exothermic peak temperatures were 125.0, 131.1, 111.4, and 125.2 degrees C, and endothermic peak temperatures were 135.8, 147.6, 118.9, and 138.4 degrees C, respectively, for H2SO4, HCl, H3PO4, and NaOH/NH4OH catalysts. The close proximity of the peak temperatures of the exothermic and endothermic reactions strongly suggests that there is potential interference of heat flow between the exothermic and endothermic reactions which may impact resin curing. (C) 2010 Wiley Periodicals, Inc.dagger J Appl Polym Sci 119: 3538-3543, 2011