The rate and extent of a synthetic sorbent containing 83wt% CaO, 14wt% MgO and 3wt% Ca12Al14O33 to react with CO2 at ~750°C was investigated in the presence of 5 or 10vol% steam. Similar to the natural sorbents and their derivatives, the rate and extent of carbonation by the synthetic sorbent was found to increase substantially with the partial pressure of steam. This improvement appears to be due to the ability of steam to promote the growth of product islands of CaCO3 in the direction normal to the surface, as well as affecting the porosity of the calcined material.