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 phenol-formaldehyde (PF) resin strength and formaldehyde emission is presented. The method relates results obtained by C-13-NMR analysis of liquid PF resins with the strength and formaldehyde emission in the resin-hardened state. Correlation of different peak ratios with experimental results allows the proposal of equations relating the sum of different C-13-NMR peak ratios with the two mentioned physical properties of the same resins in their hardened state. The equations presented appear to have some applied value in predicting physical properties of hardened industrial-type PF resins from a single C-13-NMR spectrum of the original liquid resin, as well as to render easier comparison between different PF resin formulations.