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Description: Urea-formaldehyde resins are bulk products of the chemical industry. Their synthesis involves a complex reaction network. The present work contributes to its elucidation by presenting results from detailed NMR spectroscopic studies with different methods. Besides 1H NMR and 13C NMR also 15N NMR spectroscopy is applied. 15N-enriched urea was used for the investigations. A detailed NMR signal assignment and a model of the reaction network of the hydroxymethylation step of the synthesis are presented. Due to its higher spectral dispersion and the fact, that all key reactions directly involve the nitrogen centers, 15N NMR provides a much larger amount of detail than 1H and 13C NMR spectroscopy. Symmetric and asymmetric dimethylol urea can be clearly distinguished and separated from monomethylol urea, trimethylol urea and methylene bridged urea. The existence of hemiformals of methylol ureas is confirmed. 1,3,5-oxadiazinan-4-on (uron) and its derivatives were not found in the reaction mixtures investigated here, but were prepared via alternative routes. The molar ratios of formaldehyde to urea were 1, 2 and 4, the pH-values 7.5 and 8.5 and the reaction temperature 60° C.