Deactivation of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts by carbonaceous species has been previously postulated. This mechanism, however, is difficult to prove due to the presence of long chain hydrocarbon wax product and the potential accumulation of inactive carbon on the catalyst support. Furthermore, due to the slow build-up of low quantities of inactive carbon with time on stream, the investigation of carbon deposition necessitates the use of data from extended FTS runs. In this study, the formation of carbon deposits on samples of a Co/Pt/Al2O3 catalyst, taken from a 100-barrel/day slurry bubble column reactor operated over a period of 6 months at commercially relevant FTS conditions is reported. The spent catalysts were wax extracted in an inert environment and the amount, nature and location of carbon deposits were then studied using temperature programmed hydrogenation and oxidation (TPH/TPO), energy filtered transmission electron microscopy (EFTEM), high sensitivity low energy ion scattering (HS-LEIS) and hydrogen chemisorption. TPH/TPO showed that there is an increase in polymeric carbon with time on stream which may account for a part of the observed long-term catalyst deactivation. Carbon maps from EFTEM as well HS-LEIS data show that the polymeric carbon is located both on the alumina support and cobalt. Although there is clearly an interplay of various deactivation mechanisms which may also include sintering, poisoning and cobalt reconstruction, the evidence presented shows that the polymeric carbon on the metal may be linked with a part of the longer term catalyst deactivation. (C) 2008 Elsevier B.V. All rights reserved.