A modified grain model of the reaction between calcined chalk, sulphur dioxide and oxygen has been developed, and the model has been verified by comparison with a large volume of experimental data. A chalk particle is constituted of grains that are non-porous in the uncalcined state. The pore volume of calcined chalk is distributed between macropores corresponding to the interstices between grains and micropores formed in the grains during calcination. The porous grains are assumed to be constituted of non-porous micrograins. Mass transfer in the pores takes place by molecular diffusion and Knudsen diffusion, and micrograins react with sulphur dioxide and oxygen according to a shrinking, unreacted-core mechanism. Since calcium sulphate formed by the reaction has a significantly higher molar volume than calcium oxide, micrograins will grow in volume with increasing degree of sulphation, eventually filling the micropores at a degree of sulphation of approximately 50%. Further reaction in the grains takes place according to a shrinking, partially-reacted-core mechanism, accompanied by an increase in grain volume. The only unknown parameters in the model are the tortuosity factor and the diffusion coefficient in the solid product layer encasing micrograins and grains.