In the magnesium bisulphite wood pulping process, the phase transition of magnesium sulphite hexahydrate into the trihydrate is one important reaction during the absorptive SO2 recovery from liquor burning waste gas. The kinetics of this phase transition was measured online by Raman spectroscopy. With increasing temperature the stability of the hexahydrate decreases. A decreasing pH caused faster conversion. These results allow the optimisation of the SO2 recovery process. These results are used to improve the simulation model for the recovery process in magnesium bisulphite pulping. (C) 2004 Elsevier B.V. All rights reserved.