The sulphur dioxide absorption capabilities of aqueous humic acid-fly ash mixtures were investigated. Humic acid (HA) promotes the dissolution of basic material in fly ash, resulting in the formation of humate salts. These salts absorb SO 2 effectively via an acid-base reaction resulting in formation of metal bisulphites and HA as products. The extent of fly ash dissolution in HA is a function of the HA fly ash ratio, with greater dissolution occurring as the ratio increases, until an excess of HA is present. The utilization of base from fly ash for neutralization of S0 2 is a function of its dissolution, and hence is also dependent on the HA fly ash ratio. Besides neutralization to bisulphite, HA-fly ash mixtures can also absorb SO 2 through a specific HA-SO 2(aq) interaction. However, the importance of this reaction is relatively small. On the basis of this work, it can safely be predicted that HA-fly ash mixtures would greatly outperform water-fly ash slurries in stack gas scrubbing applications.