The dissolution of CO2 in seawater in the form of bicarbonate ions is an attractive alternative to the storage in geological formations, on the conditions that the storage is stable over long times and does not harm the marine environment. In this work, we focus on the long-term chemical stability of CO2 absorbed in seawater as bicarbonate, by monitoring the physico-chemical properties of the solutions (pH, dissolved inorganic carbon and alkalinity), in six different sets of experiments on both natural and artificial seawater lasting up to three months. The bicarbonate treatment of natural seawater consists in pouring pre-equilibrated solutions obtained from the reaction of CO2 and Ca(OH)2. If the pre-equilibrated mixture does not exceed a critical threshold (ca. 1000-1500 mmol/L), the resulting bicarbonate-rich solutions can be stable for over three months.