Carbonation of hydrous minerals such as calcium hydroxide (Ca(OH)2) is an important process in environmental and industrial applications for the construction industry, geological disposal repositories for nuclear waste, and green technologies for carbon capture. Although the role of ions during the carbonation mechanism of Ca(OH)2 is still unclear, we identiﬁed the exchange of ions during the dissolution and precipitation process, by determining the change in isotopic composition of carbonation products using time-of-ﬂight-secondary ion mass spectrometry. Our samples of pure Ca(18OH)2 carbonated in air were characterized using scanning electron microscopy and Raman spectroscopy, aided by density functional theory calculations. Our results show that the carbonation process at high pH is a two-stage mechanism. The ﬁrst stage occurs in a short time after Ca(18OH)2 is exposed to air and involved the dissolution of surface Ca ions and hydroxyl 18OH groups, which reacts directly with dissolved CO2, leading to 1/3 of 18O in the oxygen content of carbonate phases. The second stage occurs within 24 h of exposure allowing a rebalance of the oxygen isotopic composition of the carbonate phases with a higher content of 16O.