Four carbon-based solid acid catalysts were prepared from single-walled carbon nanohorns (SWCNHs), oxidized SWCNHs (ox-SWCNHs), activated carbon (AC), and carbon black (CB) by hydrothermal sulfonation at 200 degrees C in an autoclave reactor. N-2 adsorption analysis suggested that sulfonation treatment leads to a drastic reduction in the number of relatively large pores with diameters greater than 20 nm in the SWCNHs and ox-SWCNHs. In addition, the BET surface area of the SWCNH was doubled by this sulfonation. These structural changes were not observed in AC and CB. The acid functional group formed on these solid catalysts by sulfonation was speculated to be -SO3H, and this was analyzed by ion-exchange titration. From the results, it was found that sulfonated SWCNHs (SO3H-SWCNHs) had the highest acid density of the four sulfonated specimens, which is a desirable property for its use as a biodiesel production catalyst. Methyl palmitate, a kind of biodiesel, was produced by the esterification of palmitic acid using these four catalysts. SO3H-SWCNHs produced the highest yield and the catalytic activity was significantly higher than that using a homogeneous sulfuric acid catalyst. Repeated esterification experiments suggested that the acid sites on SO3H-SWCNHs and SO3H-ox-SWCNHs were more stable than those on SO3H-AC and SO3H-CB. (C) 2013 Elsevier B.V. All rights reserved.