Eastern black spruce,Picea mariana, chips have been treated to high yields, at temperatures from 120° to 150°C, with sodium bisulphite solutions of pH 4 and total SO2concentrations from 0.23 to 1.92 mol L−1. The rate of sulphonation depends on the concentration of sites available for sulphonation and total SO2 concentration to the power of 0.4; it increases by approximately 70 percent for each 10°C temperature rise. It is postulated that sulphonation at pH 4 occurs by two mechanisms, one which is dependent on total SO2concentration and involves a quinonemethide intermediate and one which is independent of total SO2concentration and involves a carbonium ion intermediate.