This paper evaluates the simultaneous measurement of dissolved gases (CO 2 and O 2/Ar ratios) by membrane inlet mass spectrometry (MIMS) along the 180° meridian in the Southern Ocean. The calibration of pCO 2 measurements by MIMS is reported for the first time using two independent methods of temperature correction. Multiple calibrations and method comparison exercises conducted in the Southern Ocean between New Zealand and the Ross Sea showed that the MIMS method provides pCO 2 measurements that are consistent with those obtained by standard techniques (i.e. headspace equilibrator equipped with a Li–Cor NDIR analyser). The overall MIMS accuracy compared to Li–Cor measurements was 0.8 μatm. The O 2/Ar ratio measurements were calibrated with air-equilibrated seawater standards stored at constant temperature (0 ± 1 °C). The reproducibility of the O 2/Ar standards was better than 0.07% during the 9 days of transect between New Zealand and the Ross Sea. The high frequency, real-time measurements of dissolved gases with MIMS revealed significant small-scale heterogeneity in the distribution of pCO 2 and biologically-induced O 2 supersaturation (ΔO 2/Ar). North of 65°S several prominent thermal fronts influenced CO 2 concentrations, with biological factors also contributing to local variability. In contrast, the spatial variation of pCO 2 in the Ross Sea gyre was almost entirely attributed to the biological utilization of CO 2, with only small temperature effects. This high productivity region showed a strong inverse relationship between pCO 2 and biologically-induced O 2 disequilibria ( r 2 = 0.93). The daily sea air CO 2 flux ranged from − 0.2 mmol/m 2 in the Northern Sub-Antarctic Front to − 6.4 mmol/m 2 on the Ross Sea shelves where the maximum CO 2 influx reached values up to − 13.9 mmol/m 2. This suggests that the Southern Ocean water (south of 58°S) acts as a seasonal sink for atmospheric CO 2 at the time of our field study.