The solubility of calcite and aragonite in carbonic acid has been determined over a CO 2 concentration range from 10 -4 to 10 -3 molal and a temperature range from 10° to 70°C. The measurements were made in the absence of a gas phase in a newly developed conductometric solubility apparatus. The temperature derivative of solubility of calcite at constant CO 2 concentration varies from -10 -6 to -3 × 10 -5 molal/°C, being a maximum for the highest CO 2 concentration and temperature. The derivative of the solubility of calcite with respect to CO 2 concentration at constant temperature has a value approximately 1 at very low CO 2 concentrations and decreases as the CO 2 concentration rises. At low CO 2 concentrations, the solubility of aragonite is the same as that of calcite within the experimental errors. At a CO 2 concentration of 10 -3 molal, aragonite is about 11 per cent more soluble than calcite.