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The effect of a very high CO₂ mole fraction (27000–35000 μmol mol⁻¹) on photosynthesis and water relations was studied during the dry and the rainy season in plants of Spatiphylum cannifolium (Dryand.) Schott and Bauhinia multinervia (H.B.K.) DC. growing near natural cold CO₂ springs. Xylem water potential in plants of both species was lowered by drought, high CO₂ growth‐concentration decreasing it further in S. cannifolium. In plants of both species growing under high CO₂ concentration photosynthetic rates measured at a CO₂ mole fraction of 1000 μmol mol⁻¹ were higher than in plants growing at ambient CO₂ mole fraction and measured at 350 μmol mol⁻¹. The response was the result of a direct effect of CO₂ on the photosynthetic machinery. Changes in carboxylation efficiency in response to high CO₂ were found during the rainy season, with an increase in S. cannifolium and a decrease in B …