Using chemically defined compounds (first reaction product intermediates of the phenol-formaldehyde system), we have studied the reaction mechanisms of formaldehyde condensation in perfectly controlled conditions (temperature, stoichiometry, catalyst and pH). The results obtained with a range of techniques, e.g. high-performance liquid chromatography, C-13 nuclear magnetic resonance and chemical assay, show that the reaction kinetics and the structures of the resole end-products depend on synthetic and operating conditions.