The ability of three species of higher aquatic plants to raise pH and oxygen concentration was investigated in closed bicarbonate media differing markedly in total alkalinity (TA). Elodea canadensis Michx. alkalized bicarbonate media more than all other species, up to pH 10.4, whereas Ceratophyllum demersum L. alkalized to only 10.0 Callitriche hamulata Kütz. appeared to be a typical HCO 3 −1 non-user. In HCO 3 − users, the highest final pH was reached within TA of 0.8–2.7 mmol l −1. In E. canadensis grown in soft and hard water, no difference in alkalization capacity was found, and only a small difference was found in Ceratophyllum demersum. Significant drops in TA occurred in all species during the exposure period. The results indicate the potential alkalization in natural standing waters brought about by higher aquatic plants.