Description: The potential impacts of seawater acidification on the concentrations of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), dissolved acrylic acid (AAd) and various volatile halocarbons, including CH3Cl, CHBr3, CH2Br2, CHBr2Cl, CHBrCl2 and CH3I, were examined during a laboratory CO2 perturbation experiment for the microalgae Phaeocystis globosa and Nitzschia closterium. The microalgae were exposed to ambient CO2 conditions (390–540 µatm; 1 µatm = 0.1 Pa) and to projected concentrations for the end of the century (760–1000 µatm, high carbon (HC)). The growth rate of the two species remained unaffected by elevated CO2. Results showed a 48 and 37 % decline in the DMS concentration normalised to cell density in P. globosa and N. closterium cultures in the HC treatment compared with the ambient treatment. No significant difference was observed for DMSPp and DMSPd in the two microalgae cultures between the two CO2 levels. The mean AAd concentrations in the P. globosa culture showed a 28 % decline in the HC treatment. By contrast, the cell-normalised concentrations of AAd in the HC treatment were 45 % lower than in the ambient treatment in N. closterium cultures. No CO2-induced effects were observed for CH3Cl, CHBr3, CHBr2Cl, CHBrCl2 and CH3I, but cell-normalised concentrations of CH2Br2 in N. closterium cultures showed a 32 % decline in the HC treatment relative to the ambient level. These results show that the metabolism processes responsible for the production of climate-active gases in phytoplankton may be affected by high CO2 levels. There may be a potential delay in the responses of trace gas emissions to elevated pCO2.