Reactions of CaO, MgO, and decarbonated dolomite (CaOMgO) with CO2 and added water have been studied with the goal of understanding fundamental issues related to these materials performance as CO2 sorbents. We used a fixed bed reactor, in situ XRD, and DRIFTS to monitor the extent and kinetics of carbonation, surface reactions, and performance loss during repetitive adsorption-desorption cycles at industrial relevant conditions. From reactor and in situ XRD experiments, we found that water is essential to reach high carbonation levels (solid conversion >40%) of CaO and CaOMgO, which is in contrast to a situation where only a small fraction (<10%) of the capacity is used. Water has a more pronounced effect when applying CaOMgO as sorbent as compared to CaO, both when considering solid conversions and carbonation rates. DRIFTS shows that water together with CO2 do in fact react at the MgO surface into carbonates species. Furthermore, H2CO3 may be important for exploiting CaO and CaOMgO materials because hydrogen carbonate is observed as a surface species only during reactions with water.