Recent investigations of the relationships between the various forms of carbonic and boric acids in sea water have made it possible to delimit experimentally the solubility of calcium carbonate in the sea and to estimate quantitatively the relative effects on the solubility of variations in salinity, temperature, hydrostatic pressure, and carbon dioxide content. Although the experimental results obtained by various workers for the solubility product are not in agreement, it is at least certain that surface sea water at a temperature of 30 degrees C is saturated with calcium carbonate. A table is presented showing the comparative effects on the solubility of changes in salinity, temperature, depth, and carbon dioxide content. It is shown that ex. except for water in equilibrium with the atmosphere the most important factor controlling the solubility of CaCO 3 in sea water is the CO 2 content of the water, which, in turn, is chiefly dependent upon the nature and amount of biological activity. The order of importance of the other factors is temperature, salinity, and hydrostatic pressure. For water in equilibrium with the atmosphere, a condition probably only rarely attained, changes in temperature have the greatest effect on the solubility.