In 2021, the Ocean Thematic Centre (OTC) of the European Research Infrastructure Integrated Carbon Observation System (ICOS) conducted an international pCO2 instrument intercomparison, aimed at better understanding how different types of instrumentation for the measurement of ocean pCO2 compare to each other. During the two-week long experiment, we installed 29 pCO2 instruments in a tank at the Flanders Marine Institute (VLIZ) in Oostende/Belgium, including direct air-water equilibration systems and membrane-based flow-through instruments along with submersible sensors and instruments that are normally installed on buoys and autonomous surface vehicles. This dataset comprises an archive of data and documents collected during the study, in support of T. Steinhoff et al, “The ICOS OTC pCO2 instrument inter-comparison”, Limnology and Oceanography: Methods (in review). See the document "Description of The ICOS OTC pCO2 instrument inter-comparison data archive" (https://meta.icos-cp.eu/objects/-92T7hdhsPXNyWxZIcb0-x9u) for more information about this dataset.