The characterization of phenol-formaldehyde condensates obtained by base catalysis is described. Trimethylsilyl derivatives of the components were prepared and analysed by gas chromatography-mass spectrometry. Electron impact ionization mass spectra of methylolated dihydroxydiphenylmethane positional isomers are discussed. Prominent differences in ion abundances facilitate the identification of the compounds. Substituted trinuclear phenols among the reaction products are also described.