This study investigated the effectiveness of the magnesium coagulation process in reduction of trihalomethane (THM) precursors. The water was obtained from Lake Washington, a highly colored potable water supply which is used by the City of Melbourne, Florida. The THM concentrations in the finished water at Melbourne currently exceed the THM standard of 0.1 mg/l. For Lake Washington water, treatment varies according to seasonal changes in water quality. During the dry period, the recycle magnesium does not perform effectively as a coagulant for THM precursor removal. This is because of the high levels of magnesium in the late water at this time. During the wet period, when magnesium concentrations are very low, it is much more effective. Magnesium sulfate was found to be effective in reducing THM precursors for both types of water. There was a direct correlation observed between THMFP, TOC and color observed in treated water samples. The lack of effectiveness in THMFP, TOC and color removals by recycle magnesium was not found to be due to a lack of magnesium precipitation. Variations in rapid and slow mixing times had no effect on THM precursor removal, but did affect settling of the floc. Addition of alum as a polymer at high pH values was also very effective in reducing the THMFP, TOC and color, and increasing floc sedimentation