The structure of oligomers obtained from mono-hydroxymethylphenols in melt condensation at 120 degrees C was determined using C-13 NMR spectra in CD3OD solution. Alongside of methylene region of spectrum, valuable information was obtained from signals of aromatic carbons. Noncatalytic conditions promote the formation of dihydroxydibenzyl ethers in equilibrium with ortho- and para-benzoquinones of oxymethylene derivatives. The final methylene linked oligomers are formed, mainly, by splitting the ether intermediates with free aromatic positions. In alkaline conditions, highly nucleophilic phenoxide ions of ortho-hydroxymethyl compounds are responsible for substitution in free aromatic positions. The most favored reaction in the mixture of both hydroxymethylphenols is the formation of p,p'-methylene. In condensation of parahydroxymethyl phenol, formation of p,p'-methylene groups occurs with simultaneous release of formaldehyde. High content of alkali stabilized ortho-hydroxymethyl groups of fully substituted methylene linked oligomers determines the curing behavior of resol phenol-formaldehyde resins. The role of hemiformals in reactions was insignificant. (C) 2007 Wiley Periodicals, Inc.