The water gas shift (WGS) reaction was conducted in the presence of two natural and two synthetic CaO-based sorbents. It was shown that such sorbents can affect the WGS in two ways: (i) by catalysis of the reaction and (ii) by altering the equilibrium position by abstraction of CO2 from the gas phase. It was shown that CaO can significantly enhance the production of H2 during the WGS reaction; however, a trade-off between the production of H2 and contamination of the product gas with CO2 (the “CO2 slip”) has to be made. It was found that CaO catalyzes the WGS reaction. The carbonation reaction was very close to thermodynamic equilibrium, even at small contact times at 650 °C. However, the concentration of H2 was significantly below that predicted from equilibrium considerations. In our experiments, once the sorbent had been fully carbonated, it was regenerated by heating to release the CO2 so that it could be reused. In such a cyclic experiment, calcium magnesium acetate, a synthetic sorbent, was the best...