Abstract. The physiological performance of two coccolithophore species, Emiliania huxleyi and Coccolithus braarudii, was investigated during long-term exposure to elevated pCO₂ levels. Mono-specific cultures were grown over 152 (E. huxleyi) and 65 (C. braarudii) generations while pCO₂ was gradually increased to maximum levels of 1150 μatm (E. huxleyi) and 930 μatm (C. braarudii) and kept constant thereafter. Rates of cell growth and cell quotas of particulate organic carbon (POC), particulate inorganic carbon (PIC) and total particulate nitrogen (TPN) were determined repeatedly throughout the incubation period. Increasing pCO₂ caused a decrease in cell growth rate of 9% and 29% in E. huxleyi and C. braarudii, respectively. In both species cellular PIC:TPN and PIC:POC ratios decreased in response to rising pCO₂, whereas no change was observed in the POC:TPN ratios of E. huxleyi and C. braarudii. These results are consistent with those obtained in shorter-term high CO₂ exposure experiments following abrupt pertubations of the seawater carbonate system and indicate that for the strains tested here a gradual CO₂ increase does not alleviate CO₂/pH sensitivity.