Understanding mechanisms and kinetics of mineral carbonation reactions relevant to sequestering carbon dioxide as a supercritical fluid (scCO 2 ) in geologic formations is crucial to accurately predicting long-term storage risks. Most attention so far has been focused on reactions occurring between silicate minerals and rocks in the aqueous dominated CO 2 -bearing fluid. However, water-bearing scCO 2 also comprises a reactive fluid, and in this situation mineral carbonation mechanisms are poorly understood. Using in situ high-pressure X-ray diffraction, the carbonation of brucite [Mg(OH) 2 ] in wet scCO 2 was examined at pressure (82bar) as a function of water concentration and temperature (50 and 75°C). Exposing brucite to anhydrous scCO 2 at either temperature resulted in little or no detectable reaction over three days. However, addition of trace amounts of water resulted in partial carbonation of brucite into nesquehonite [MgCO 3 ·3H 2 O] within a few hours at 50°C. By increasing water content to well above the saturation level of the scCO 2 , complete conversion of brucite into nesquehonite was observed. Tests conducted at 75°C resulted in the conversion of brucite into magnesite [MgCO 3 ] instead, apparently through an intermediate nesquehonite step. Raman spectroscopy applied to brucite reacted with 18 O-labeled water in scCO 2 show it was incorporated into carbonate at a relatively high concentration. This supports a carbonation mechanism with at least one step involving a direct reaction between the mineral and water molecules without mediation by a condensed aqueous layer.