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-formaldehyde (MF) resins strength is presented. The method relates results obtained by NMR analysis of liquid MF resins with their strength 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 MF resin from the C-13-NMR spectrum of the liquid MF resin.