The effects of calcium and magnesium ion concentrations in artificial seawater (ASW) on the equilibrium state of the system H2CO3-H2O have been tested. The measured first and second apparent dissociation constants of carbonic acid, K¡¦1 and K¡¦2, increase with an increase in the concentrations of calcium and magnesium on ASW. The total and free activity coefficients of HCO3- and CO23- at the ionic strength of 0.718 and temperature of 25¢J decrease as a function of Mg:Ca concentration ratios in ASW. The results also show that the calcium ion has a greater effect on the apparent dissociation constants of carbonic acid than magnesium, suggesting that the calcium ion associates more strongly with bicarbonate and carbonate than the magnesium ion does.