The influence of acid treatment on carbon nanotubes (CNT) supported cobalt catalysts for Fischer-Tropsch synthesis (FTS) is discussed in this paper. CNTs were first treated with 30 wt.% HNO3 at 25 and 100 degrees C for 14 h. Cobalt catalysts supported on fresh and acid treated carbon nanotubes were prepared using the incipient wetness impregnation method with a cobalt loading of 10 wt.% The catalysts were extensively characterized by BET, XRD, TPR and TEM, and Raman spectroscopy. The TEM analyses of the acid treated support catalysts showed that the major parts of the cobalt particles were homogenously distributed inside the nanotubes. Raman I-D/I-G band intensity ratios as an indication of the quality of carbon nanotubes for catalyst supports, increased with acid treatment. The FTS activity (g HC produced/g cat./h) and selectivity (the percentage of the converted CO that appears as a hydrocarbon product) of the catalysts were assessed and compared with the as-prepared CNT supported 10 wt.% cobalt catalyst using a fixed bed micro-reactor. The acid treatments at 25 and 100 degrees C respectively, (a) increased the BET surface area by 18% and 25%; (b) decreased the cobalt particle size and increased the cobalt dispersion; (c) increased by 10 and 50% the reducibility of the catalysts and (d) increased the FTS activity and %CO conversion by 36 and 114%. Finally, the product selectivity showed a distinct shift to lower molecular weigh hydrocarbons. (c) 2008 Published by Elsevier B.V.