The steam-enhanced carbonation reaction of CaO with CO2 is a widely observed phenomenon, but its mechanism is unclear. This study proposed that OH formation caused by the dissociation of H2O molecules may explain enhanced carbonation. The relationship of oxygen vacancies with OH at different temperatures and steam fractions was established and integrated into a new carbonation model. This new model included a simplified rate equation model to describe product island formation and a multi-ion diffusion model that describes product layer growth. Experiments on carbonation and sorption- enhanced water gas shift reactions in a laboratory-scale fluidized bed reactor were conducted to validate the idea of OH formation. The new developed models were validated by thermogravimetric analysis. (C) 2013 Elsevier Ltd. All rights reserved.