The reaction of kraft lignin in an alkaline medium was studied using a microreactor set. Chemical changes in reacted kraft lignins that include ash content, Klason lignin, acidsoluble lignin and sugars were studied. Structural characteristics of these lignins were also determined in terms of phenolic-hydroxyl, aliphatic-hydroxyl, methoxyl, M-w, M-n and polydispersity. The techniques employed were HPLC, UV spectroscopy, FTIR spectroscopy, proton nuclear magnetic resonance spectroscopy and organic gel permeation chromatography (GPC). The effects of temperature and reaction time on lignin properties were studied using response surface methodology. The reaction temperature ranged from 116 to 180 degrees C and the reaction time ranged from 18 to 103 min. The obtained response surfaces show that both factors affected lignin properties within these ranges. The phenolic and aliphatic-hydroxyl content and the number of active sites increased when the treatment severity was increased. Weight-average molecular weight (M-w), number-average molecular weight (M-n) and solid-yield percentage decreased when the treatment severity was increased. (C) 2011 Elsevier Ltd. All rights reserved.