A method based on the use of C-13-NMR relative peak intensity ratios for different characteristic chemical groups, known or supposed to contribute to melamine-urea-formaldehyde (MUF) resin strength and formaldehyde emission, is presented. The method relates results obtained by C-13-NMR analysis of liquid MUF resins with their strength and formaldehyde emission in the resin hardened state. Sets of correlation equations are presented which can be used to predict some of the physical properties of a hardened MUF resin by studying the C-13-NMR peak ratios of well-defined chemical groups in the liquid MUF resin. A true class of MUF resins, with characteristics all of their own, appear only to exist in the copolymers in which the mass ratio of M : U is in the approximate range 55 : 45 to 34 : 66, mass ratios outside this range resulting in resins which behave almost as pure melamine or as pure urea resins. Contrary to what was found for MUF glue mixes in which lower condensation pre-resins are used as scavengers and accelerators, in pure MUF resins, the ratios of NMR peaks of the downfield substituted and unsubstituted triazine and urea signals, while still important, do not exclusively dominate the correlation equations, other chemical groups such as methylene (Me), methylol (Mo), and methylene ether groups assuming considerable importance.