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Abstract: In April 2006, diurnal variations of carbon dioxide (CO2) and oxygen (O2) in the surface water east of Gotland in the Baltic Sea were investigated with a unique multitechnology approach. Several parameters were measured simultaneously providing an overview of the CO2 system. Moored instruments were continuously recording partial pressure of CO2 in the surface water (pCO2 w), currents, mixing, waves, salinity, temperature and O2. Measurements of total alkalinity (A T) and dissolved inorganic carbon (C T) were taken from R/V Skagerak. These measurements were converted to pCO2 w to support the continuous pCO2 w data and also calculate the air–sea exchange of CO2. Additionally, the time derivatives of O2 and C T concentrations in the water were determined using incubations and a Productivity Autosampler (PA). O2 and pCO2 w were significantly anti-correlated and periods dominated of either biological processes, mixing, air–sea exchange or a combination of these were detected. O2 and pCO2 w had a daily cycle and variations occurred on the 1h time scale. In April 2006, the sea was a CO2 sink and the averaged parameterized air–sea exchange was −1.0±0.6mmolm−2 h−1. [Copyright &y& Elsevier]