An autonomous system for the recording of the surface water pCO 2 in the Baltic Sea was complemented by a module for continuous O 2 measurements. An optical O 2 sensor was used and in analogy to the determination of the pCO 2 , the pO 2 was determined in the headspace of an equilibrator. By this procedure the automatic periodic calibration was considerably facilitated and biofouling was avoided. Uncertainties were associated with the use of a vented equilibrator. Due to the continuous pressure equalization with the ambient atmosphere, the headspace partial pressure of any gas (e.g. pO 2 , pCO 2 ) reflects a steady state that may deviate substantially from the equilibrium. To calculate the true equilibrium pO 2 and thus the O 2 concentration in the water sample, we derived an equation that is based on the measurement of the headspace O 2 mol fraction, x O 2 , and on the assumption that the surface water was saturated with atmospheric N 2 and Ar. Model calculations indicated that deviations from this assumption are less then 2% for most of the year. The combined pO 2 /pCO 2 measurement system was deployed in July 2005 on a cargo ship that commutes regularly between the southwestern Baltic Sea and the Gulf of Finland. First results showed a reasonable agreement with O 2 data obtained at a measurement platform in the vicinity of the ship's track. The spatial resolution of the O 2 measurements was similar to that of the pCO 2 system and allowed the identification of small scale (b 1 nautical mile) biogeochemical and hydrographical inhomogeneities.