We have measured the carbon isotopic composition of particulate organic matter suspended in surface waters (POM) between 59¿S and 30¿S in the SW Indian Ocean during the austral summer. In an attempt to further document the pattern and causes of covariance between POC-&dgr;13C and [CO2aq], we concurrently measured surface water pCO2, temperature, salinity, nitrate concentration, POM concentration, chlorophyll a?, and the &dgr;13C of total dissolved inorganic carbon, While we found the previously reported general negative correlation between POC-&dgr;13C and [CO2aq], we also observed a prominent maximum in POC-&dgr;13C in the region immediately north of the Subtropical Convergence, coinciding with a maximum in [POM] and chlorophyll a?, and with a minimum in pCO2. The increase in POC-&dgr;13C between 59¿S and the Subtropical Convergence is consistent with the trend expected if [CO2aq] were the main factor controlling the isotopic composition of POM. In contrast, data from the region north of the Subtropical Convergence clearly illustrate that POC-&dgr;13C can also vary independently of [CO2aq], as a 5 per mil decrease in POC-&dgr;13C was found in a region characterized by nearly constant [CO2aq]. We review several physiological factors which may account for these observations and discuss their implications for paleoceanographic reconstruction of [CO2aq] from the carbon isotopic composition of sedimentary organic matter. ¿ American Geophysical Union 1993