The submitted datasets contain surface seawater partial pressure of carbon dioxide (pCO2) values measured with Kongsberg Contros/4H-Jena HydroC-FT membrane-based sensors. These sensors were integrated on the FerryBoxes installed on commercial vessels travelling in the North Sea and maintained by the Institute of Coastal Ocean Dynamics at the Helmholtz-Zentrum Hereon, Germany. The pCO2 data were reprocessed from the raw data and corrected for sensor post-calibration where this was available. The instrument produces a result every second. 20-second averages are used for calculations and reported. Temperature and salinity results are also provided where available. These were measured from the underway with Falmouth Scientific Instruments / Teledyne Instruments sensors also integrated with the FerryBox. The sensors were regularly maintained and occasionally replaced. Each sensor replacement results in a new dataset within this publication series. The files are named using the ship name and date of the first measurement.