A synthetic, Ca-based solid sorbent, showed a marked increase in its ultimate uptake for CO2 at temperatures in excess of 750 °C when the concentration of CO2 was increased during carbonation in a fluidised bed. In contrast, the uptakes by natural sorbents, e.g. dolomite, were relatively insensitive to the concentration of CO2. It is apparent that the rate of reaction of the synthetic sorbent falls to zero once the small pores within the grains, of which the particle is composed, have largely filled and a thin layer of product has been deposited around each grain. To quantify this effect, theory has been developed in which the mechanical work required to disrupt the layer of product is taken into account. The resulting model for the overall uptake correlates experimental measurements well, except in circumstances where carbonation times are so long that sintering introduces gross changes in the morphology of the layer of product. The explanation for why the overall conversion of the synthetic sorbent is dependent on the concentration of CO2, whilst the conversion of dolomite is insensitive to [CO2], is attributed to differences in the yield stress, ?Y, needed to disrupt the layer of product formed in the two materials. It was found that the value of ?Y for dolomite is about an order of magnitude larger than that for the synthetic sorbent. The behaviour of the synthetic sorbent in response to increasing concentrations of CO2 makes it an attractive solid for the separation of CO2 from, e.g. the flue gases arising from combustion. By subsequently calcining the resulting solid, a pure stream of CO2 can be produced whilst the sorbent is regenerated for further use.