In an effort to investigate the controlling factors behind Holocene delta C-13(org) changes in East Antarctica we report high-resolution down-core records of bulk organic matter carbon isotopic ratios (delta C-13(org)), diatom census counts, total organic carbon, and biogenic silica content taken from one core recovered in the Adelie Trough. A good correspondence between the delta C-13(org) record and records of small/large and pennate/centric ratios in many of the core segments indicates that diatom species composition affected Holocene delta C-13(org) variations, possibly via diatom shape and size effects. Variations in the surface water CO2 concentration and in the isotopic composition of the source during the Holocene cannot be ruled out although they cannot explain rapid and large-amplitude delta C-13(org) changes. Within the limit of our investigation, our results argue against active carbon acquisition through a carbon concentration mechanism as evidenced in low-latitude upwelling systems. The bulk organic matter thus represents a mixing of diatom taxa having different fractionation affinity to aqueous CO2. Our results confirm previous evidence which demonstrates that delta C-13(org) down-core records should be used with great caution to reconstruct past CO2 content in surface waters.