The effect of irradiance on the tolerance to high pH of the two marine protists Heterocapsa triquetra and Nitzschia navis-varingica were investigated by conducting a series of pH drift experiments. In order to select appropriate levels of irradiance for the pH drift experiments, the photosynthetic activity and the growth rates of the two species were investigated at irradiances from 5 to 250 µmol photons m^-2 s^-1. Irradiances of 20, 35, 80 and 250 µmol photons m^-2 s^-1 were subsequently selected for further studies, because they represented conditions from severely light limited to fully light saturated with respect to photosynthesis and growth. At all four light levels, the pH limits for growth were 9.29-9.42 for H. triquetra and 9.54-9.77 for N. navis-varingica. The pH limit for growth was not positively correlated to irradiance in either of the species. We therefore conclude that low light does not reduce the pH tolerance of the protists studied