We studied the enhancement of lignin reactivity in an alkaline medium, using sodium hydroxide in a microreactor set. The chemical composition and structural characterization of the reacted lignosulfonate in terms of the phenolic hydroxyl groups, aromatic protons, weight-average molecular weight, number-average molecular weight, and lignosulfonate content of all reacted lignins were determined. The techniques that we used were ultraviolet spectroscopy, proton nuclear magnetic resonance spectroscopy, and aqueous gel permeation chromatography. Using response surface methodology, we studied how the temperature and reaction time affected the lignin properties. The reaction conditions were temperatures between 116 and 180 degrees C and reaction times between IS and 103 min. Modeled response surfaces showed that the two factors affected the lignin properties within the studied ranges. The phenolic hydroxyl groups, aromatic protons, and lignosulfonate content increased when the severity of the treatment increased. The weight-average molecular weight, number-average molecular weight, and solid yield (%) decreased when the severity of the treatment increased. The reactivity of the modified lignins was studied with a formaldehyde reactivity test: more severe conditions produced greater improvements in the formaldehyde reactivity. (c) 2006 Wiley Periodicals, Inc.