The mineralogy and chemistry of marine evaporites sets limits on excursions of the composition of sea water which may have occurred during the Phanerozoic. The permitted volume occupied by Phanerozoic sea water in log mSO4- — log mca+2. —log HCO3- space has roughly the shape of a doubly terminated pyramid. Present-day sea water lies near the center of the permitted volume. The maximum permitted concentration of all the major components in sea water is modest. This is particularly true of Ca+2 and HCO3-; the oceans have apparently been able to dispose effectively of river alkalinity during the entire Phanerozoic.