The concentration dependence of the molecular association of 2,4,6-trihydroxymethylphenol (THMP) as a model compound for phenol-formaldehyde resin was studied. C-13-NMR spin-lattice relaxation time (T-1) measurements indicated that the molecular interactions were influenced by the concentration of THMP. Plots of T, versus the concentration revealed an inflection point around 1.2 mol/L. Molecular orbital calculations of the THMP molecules implied the following features of the molecular interactions: in the concentration range below 1.0 mol/L, THMP existed as a single molecule, and then at a higher concentration, self-association started. At concentrations higher than 1.5 mol/L, almost all the THMP molecules were bimolecularly associated, and the formation of larger clusters was initiated. (c) 2006 Wiley Periodicals, Inc.