Eastern black spruce, Picea mariana, chips have been treated to high yields, at temperatures from 72° to 150°C with sulphite solutions of pH 7 and total SO2 concentrations from 0.1 to 1.0 mol L-1. The rate of sulphonation of black spruce chips is directly proportional to the product of concentration of sites available for sulphonation and the total SO2 concentration. The energy of activation for sulphonation is 62.6 ± 2.3 kJ mol-1. The rate of lignin dissolution at pH 7 is dependent only upon the temperature. High total SO2concentrations are necessary for the efficient production of high strength ultra-high yields pulps. It is postulated that sulphonation at pH 7 is governed by a rate limited addition of bisulphite to quinone methide intermediates formed from phenolic lignin units.