Gas-phase magnesium hydroxide carbonation processes were investigated at high CO2 pressures to better understand the reaction mechanisms involved. Carbon and hydrogen elemental analysis, secondary ion mass spectrometry, ion beam analysis, X-ray diffraction, and thermogravimetric analysis were used to follow dehydroxylation/rehydroxylation/carbonation reaction processes. Dehydroxylation is found to generally precede carbonation as a distinct but interrelated process. Above the minimum CO2 pressure for brucite carbonation, both carbonation and dehydroxylation reactivity decrease with increasing CO2 pressure. Low-temperature dehydroxylation before carbonation can form porous intermediate materials with enhanced carbonation reactivity at reduced (e.g., ambient) temperature and pressure. Control of dehydroxylation/ rehydroxylation reactions before and/or during carbonation can substantially enhance carbonation reactivity.