The assessment of the effects of Ocean Acidifi cation (OA) on marine life has received increasing attention in recent marine research.On a mesocosmic scale, the CO 2 levels in seawater can be manipulated to evaluate experimentally the consequences of OA on marine organisms (vertebrates and invertebrates).An ideal manipulation of carbonate chemistry should mimic exactly the changes to carbonate chemistry, which will occur in years to come.Although some methods have been described in the literature, here we describe in detail a simple, robust and inexpensive system to produce CO 2 -enriched seawater by bubbling the seawater with air-CO 2 mixtures.The system uses mass fl ow controllers (MFC) to blend atmospheric air with pure C O 2 to produce two pCO 2 levels.The air-CO 2 mixtures are delivered continuously to seawater equilibration reservoirs, a nd simultaneously to an infrared CO 2 analyser to verify CO 2 levels in the air-CO 2 mixture delivered to the equilibration tanks.We monitored both pH and total alkalinity in the equilibration reservoirs over a period of one year in order to document the long-term performance of this system for simulating the future carbonate chemistry of seawater in a coastal laboratory.System performance was suffi cient to maintain three contrasting (e.g., 397, 709 and 1032 matm) and relatively constant (the coeffi cient of variability was 11 %, 9 % and 9 % respectively) seawater pCO 2 during a year-long monitoring.