Four catalysts (H2SO4, HCl, H3PO4, and NaOH/NH4OH) were studied in the preparation of melamine modified urea–formaldehyde (UFM) resins. 13C-nuclear magnetic resonance spectroscopic analysis of the UFM resins at different synthesis stages revealed the polymer structure and detailed reaction mechanism. Three acidic catalysts (H2SO4, HCl, and H3PO4) enhanced the resin polymerization through the formation of various contents of methylene, ether linkages, and urons. H3PO4 yielded the most terminal ether linkages at the first stage and enhanced polycondensation by depleting all free urea and glycols to form the most linear methylene linkages NHCH2NH in the end. However, at the initial synthesis stage, NaOH/NH4OH catalyzed the formation of UFM prepolymer to a limited extent with a large amount of free urea left, and therefore produced the final polymer with relatively more substituted methylolureas and linear ether linkages. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40644.