This dataset contains repeated measurements of seawater pH and total alkalinity recorded in the exposure mesocosms, the tanks holding the oysters exposed to five treatments (unamended control, dissolved alkalinity addition with NaOH and CaCl2 at +250 or +500 µmol/kg, and olivine-based coastal enhanced silicate weathering at +250 or +500 µmol/kg) over a 64-day chronic exposure. These treatments mirror those established in the alkalinization mesocosms. Measurements span three sampling intervals, 8 May to 27 May 2024, 29 May to 17 June 2024 and 19 June to 10 July 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.