M odifications of the EDTA titration of total hardness have been evaluated for use ,n determining magnesium in sea water. In this procedure EDTA is the titrant, Eriochrome Blue Black B the indicator, and ethanolamine the buffer. Various conditions necessary for accurate sea water titrations have been established. A mean magnesium-chlorinity ratio of 0.06689 was obtained when 30 sea water samples were titrated.