The solid residues of nine urea-formaldehyde (UF) resins after hydrolytic treatment at pH 4 and 86-degrees-C for 20 h and the solid residues of one UF resin after eight different hydrolytic treatments were examined by C-13-CP-MAS NMR. The relative stability toward hydrolysis of each structural component in each UF resin was established for various hydrolytic conditions. In general, UF resins prepared from starting mixtures with a formal F/U molar ratio of 1.00 show a high degree of stability toward hydrolysis. Cross-linking methylene linkages in the UF resins show a higher susceptibility to hydrolytic treatments at pH 4 and 86-degrees-C than do linear methylene linkages. UF resins prepared with an F/U molar ratio of 2.00 are susceptible to hydrolysis at pH 4 and 86-degrees-C. Dimethylene ether linkages, methylols attached to tertiary amides, and poly (oxymethylene glycol) moieties are probably the main formaldehyde emitters in UF resin products.