The results of an existing one-dimensional diagnostic model that calculates the fugacity of CO2 (fCO2) in the surface layer of the southern ocean were compared with in situ observations from different ocean sectors and seasons. Our model is based on the translation of monthly variations of constraints fields into surface water fCO2 variations, and was used to assess the CO2 uptake of the southern ocean. In situ observations are useful to verify the model results and were here applied to improve the estimation of the CO2 uptake of the southern ocean south of 50°S. The model reproduces the fCO2 distribution in both Pacific and Indian sectors of the southern ocean satisfactorily, the mean deviation being only 5 matm. This discrepancy requires only a minor modification of the CO2 uptake calculated by the model for that area. By contrast, the model strongly underestimates the fCO2 levels in early spring and early winter in the Weddell gyre. This indicates that the CO2 uptake by the Atlantic sector of the southern ocean as calculated by the model, amounting to 0.47 GtC yr−1, should be reduced, possibly by about half of this value. The reason for this mismatch lies in the use of climatological physical constraints by the model, that do not sufficiently well describe reality. Partly, the mismatch is also caused by a difference of seasonal stage between the model which reflects climatological conditions and the real ocean which is affected by interannual variability. Based on this study it is concluded that the CO2 uptake of the southern ocean south of 50°S is likely to lie somewhere between 0.6 and 0.7 GtC yr−1 for the 1990s, which is a high value compared to estimates from other investigations.