To measure concentrations of CO2 in the ocean surface and sub-surface waters, a whole new generation of sensors and instruments entered the market in recent years. An intercomparison of all these systems seemed necessary to the Ocean Thematic Centre (OTC) of the European research infrastructure ICOS (in full: Integrated Carbon Observation System). Hence, all interested scientists, operators and manufacturers of pCO2 sensors are kindly invited to participate in the first intercomparison exercise of ocean pCO2 instruments, held between 24 August and 4 September 2020 at the Flanders Marine Institute VLIZ in Ostend, Belgium.