Abstract Oxy-fuel combustion is a promising CCS technology which is being demonstrated prior to commercialisation. While the flue gas in oxy-fuel combustion is concentrated in CO 2 , it contains impurities such as SO 2 . The elimination of SO 2 can provide a clean CO 2 stream ready for storage. This paper is to understand the absorption of SO 2 in scrubbing relevant to those used in oxy-fuel technology. Steady state experiments were conducted in a continuous well stirred reactor to understand the absorption rate of SO 2 /CO 2 into a total concentration of 0.28 M of mixtures of NaHSO 3 and NaHCO 3 simulating liquids formed by scrubbers using NaOH as the reagent at solution pH values from 4 to 7 with the exiting gas concentrations of SO 2 from 19 ppm to 1500 ppm and a constant CO 2 concentration of 70%. Online measurement included gas phase SO 2 and liquid pH, and offline measurement included CO 2 (aq), HCO 3 − , S (IV), SO 3 2− and S (VI) after each experiment. Three aspects investigated were the impacts of pH on the solution chemistry, the significance of solution pH and the concentration of gas phase SO 2 on the absorption rate of SO 2 . The total sulphur concentration in liquid was found to be related to the effectiveness of Na + . The effective ratio of Na + can be defined as the total sulphur to Na + ratio and this effectiveness ratio of Na + is pH dependent. At pH + is 99% effective. It reduces dramatically from 99% at a pH 5 to less than 15% at pH above 7. With regard to carbon based species also absorbed, super saturation of CO 2 (aq) was observed at pH > 5.5. The concentration of HCO 3 − increases dramatically above pH 6 and below this pH, the concentration of HCO 3 − is negligible. The absorption rate of SO 2 was found to increase with pH with some increase with the concentration of SO 2 . The operational pH window for scrubbing may be defined by an upper limit pH where the absorption rate of SO 2 starts to decreases from the maximum absorption rate of SO 2 and the lower limit pH where the absorption rate of SO 2 reduces to half of the maximum absorption rate of SO 2 . Both the upper limit and the lower limit decrease initially and stay stable with the concentration of SO 2 . This decrease is caused by the reversible reaction of the hydrolysis of SO 2 and confirmed by equilibrium experiments of SO 2 and sodium solutions. Operation within region 2 (pH 5–6) is recommended, depending on the scrubber design. The operation exit pH of the produced liquid can be varied within the region. The absorption rates of SO 2 obtained in the steady state experiments were predicted by a model based on the instantaneous reaction assumption. This model generally overestimates the absorption rates of SO 2 at pH values below 6 indicating a kinetic limitation of SO 2 and water reaction at low pH values. The analysis on the controlling regions indicates that the gas side mass transfer resistance decreases with the concentration of SO 2 . Liquid side resistance becomes more important at a lower pH and a higher concentration of SO 2 .