Description: We present measured and estimated seawater physiochemical parameters at CO2 vents and ambient pH sites along the coast of Ischia (Italy) across water depths from 1 m to 40 m. We characterized the physical and chemical parameters in four CO2 venting sites (vent 1 to vent 4) and reference sites with ambient pH with no venting activity (two reference sites for each CO2 vent). SeaFETTM Ocean pH sensors (Satlantic) were deployed to quantify variation in pH at the CO2 vents and their corresponding ambient pH sites at the same depths where benthic surveys were performed from May to October 2019. Before deployment, the SeaFETs were calibrated with ambient pH water. The mean offset between calibration samples and calibrated SeaFET pH was ± 0.006 units (n= 44 water samples), indicating a high-quality pH dataset. The pH sensors were deployed in the recently discovered CO2 vents (Vent 2, Vent 3, Vent 4) and one of the corresponding reference areas with no visible vent activity during the same period (ambient 2a, ambient 3a, ambient 4a). The pH and seawater physicochemical parameters for vent 1 and their corresponding ambient sites (site Castello Aragonese) are also included here. This data from the Castello Aragonese site was originally reported by Kroeker et al., 2011. Discrete water samples were collected using Niskin bottles at the vent and reference areas with ambient pH to measure: i) the carbonate system parameters during the pH sensor deployment, and ii) the dissolved inorganic nutrients. Salinity was measured using a CTD (CTD Sea-Bird Electronics SBE 19 Plus Seacat). Discrete water samples for total alkalinity (AT) were collected within 0.25 m of the pH sensors using standard operating protocols. Precision of the AT measurements of CRMs was 〈 2 μmol kg-1 from nominal values. AT and pHT along with in situ temperature and depth were used to determine the remaining carbonate system parameters for each sampling period using the R package seacarb v3.2.12 (Gattuso et al., 2023).