Mass production of microalgae generally requires the injection of CO2 into open ponds or photo-bioreactors. The present study compares the CO2 injection efficiency into seawater of a porous stone air diffuser and a venturi. CO2 was injected at flow rates of 400, 700 and 1000 standard mL/min and 4, 7 and 10 standard L/min into a small and a large pond, respectively until the pH decreased from 7.8 to 6.8. No significant differences in CO2 injection efficiency between the three CO2 flow rates (p > 0.05) were observed; however, CO2 injection efficiency with venturi was about 100% (p < 0.05) higher than that of the air diffuser. Therefore, it is possible to both reduce the cost and increase the effectiveness of CO2 dissolution in seawater by using venturi operated at a lower flow rate, i.e. 400 standard mL/min in a small pond and 4 standard L/min in a large pond. (C) 2011 Elsevier Ltd. All rights reserved.