A recently discovered coupling agent, hydroxymethylated resorcinol (HMR), based on resorcinol-formaldehyde, can greatly enhance wood-to-epoxy resin bond durability in exterior applications. However, for PIMR to be most effective, it needs to be prepared a few hours before it is applied to the wood surface. In this study, carbon-13 nuclear magnetic resonance (NMR) spectroscopy was used to monitor composition of HMR as a function of time to characterize which chemical groups are present in solution when HMR is applied. A quantitative assessment of formaldehyde-derived groups required the use of 99% C-13-enriched formaldehyde. Hydroxymethyl groups, primarily attached to the 4-position of resorcinol, and hemiformal groups formed very quickly. Signals from methylene linkages between resorcinol rings began to appear 20 min into the reaction. Formaldehyde was consumed quickly; 95% was bound to resorcinol rings within 1.7 h. By 3 h, 16% had been converted to methylene linkages, and by 8.3 h, 40% was converted. Another set of NMR experiments was used to monitor the dependency of peak positions of resorcinol solution as a function of pH. These experiments showed significant effects, especially between pH 7.7 and 9.1, which explains chemical shift changes observed during the HMR reaction. (C) 2000 John Wiley & Sons, Inc.