C-13 cross-polarization/magic-angle spinning nuclear magnetic resonance spectroscopy has been used to investigate urea-formaldehyde (UF) resins prepared at room temperature from reaction mixtures containing N,N’-dimethylolurea or various pretreated paraformaldehyde agents as a source of CH2 groups. Resins prepared from reaction mixtures with various ’equivalent’ formaldehyde-to-urea molar ratios and various pH values and reactant concentrations were examined. Structural changes in the UF resins that accompany variations in these parameters, and the dependence of the UF resin structures on the solubilities of N,N’-dimethylolurea or various pretreated paraformaldehyde agents in water are discussed along with the relative rates of various reactions at different pH values. Comparisons are made among the UF resins prepared from formalin, N,N’-dimethylolurea or various pretreated paraformaldehyde agents as a source of CH2 groups in the resins. Valuable information on the mechanisms and reactivities of various reactions in the complicated UF resin system was drawn from these comparisons.