This dataset contains repeated measurements of seawater pH and total alkalinity recorded in the alkalinization mesocosms, a set of tanks without oysters that received the same alkalinity treatments as the exposure mesocosms reported in a companion dataset. Total alkalinity was raised in three alkalization cycles to reach target increases of +250 or +500 µmol/kg over unamended seawater, using either dissolved alkalinity addition (NaOH and CaCl2) or olivine-based coastal enhanced silicate weathering. The three cycles ran 17 April to 7 May 2024, 8 May to 27 May 2024 and 30 May to 19 June 2024. Measurements were taken predominantly in triplicate per treatment and sampling day. Seawater pH was measured with a pH electrode and total alkalinity by open-cell potentiometric titration, both on a Metrohm 855 Robotic Titrosampler.