Sodium hydroxide was added to carbonate-free sea water and the point at which Mg(OH)2 precipitated was detected optically. At this point the hydroxyl ion activity was calculated from the measured pH of the solution and the magnesium ion activity was calculated from the activity product for Mg(OH)2. From a knowledge of the magnesium ion activity and other data, it was estimated that about 3% of the total magnesium in the sea water was in the form of MgSO4.