The structural modification of softwood ammonium lignosulfonate by phenolation was studied. A central factorial design was applied to determine the influence of reaction conditions (temperature, time and lignosulfonate content) on the properties of phenolation products. The reaction was monitored through the concentration of free phenol determined by gas chromatography (GC). The characterization of phenolation products was accomplished by gel permeation chromatography (GPC), fourier transform infrared (FTIR) and H-1 nuclear magnetic resonance (H-1 NMR). Empirical models were developed to predict phenol conversion and adduct formation according to operating conditions. The optimum phenolation conditions, attained by means of a response surface method, were found to be 120degreesC; 160min and 30% lignosulfonate content. (C) 2004 Elsevier Ltd. All rights reserved.