A method based on the use of C-13-NMR relative peak intensity ratios for chemical groups known to contribute to melamine-urea-formaldehyde (MUF) and melamine-formaldehyde (MF) resin strength and formaldehyde emission is presented. The method relates results obtained by C-13-NMR analysis of liquid MUF and MF resins with their strength and formaldehyde emission in the hardened state. Correlation of different peak ratios with experimental results showed that, contrary to other formaldehyde-based resins, the NMR analysis for the MUF and MF resins needs only to take into account the triazine/substituted triazines and the urea/substituted ureas peak ratios to allow the proposal of equations correlating a single spectrum of the liquid resin with the physical properties of the boards bound with the same resin in its hardened state. Correlation equations are developed for both the case in which variation of the amine : formaldehyde molar ratio is directly induced at the resin preparation stage as well as the case in which it is induced at the glue-mixing stage, the latter by addition of different types of resin accelerators and resin scavengers.