The dehydroxylation of brucite was determined in the presence of NaCl-H 2 O fluids containing 0, 2, 6, 11, 14, 18, 30, and 38 mol% NaCl (0.0, 5.5, 17.0, 28.8, 34.5, 41.7, 58.2, and 66.6 wt% NaCl) to approximately 2500 bars and approximately 700 degrees C by high-pressure differential thermal analysis (HP-DTA). At pressures below 150 bars, brucite dehydrates in two separate reactions involving at least one intermediate phase. The dehydroxylation reaction of brucite at pressure to approximately 2 kbars follows InP(bars) = 22.23-13620/T(K). In the presence of NaCl-H 2 O fluids, the dehydroxylation temperature is lowered because of the reduction of the activity of H 2 O in these fluids. At 1 kbar pressure, mixing in the NaCl-H 2 O fluids is virtually ideal, assuming a nearly complete association of NaCl, but at 2 kbars, the dissociation of NaCl is substantial. The phase relations in the system MgO-NaCl-H 2 O show five univariant reactions emanating from an invariant assemblage, periclase + brucite + halite + liquid + vapor, located at 565+ or -5 degrees C and 440+ or -30 bars.