The catalytic activity of single-wall carbon nanohorns (SWNH) as counter electrodes (CE) of dye-sensitized solar cells (DSC) was studied for the iodide/triiodide redox reaction. The catalytic activities of SWNH and high surface SWNH (HS-SWNH) obtained by partial oxidation of SWNH were assessed based on charge-transfer resistances (R-ct) and current-voltage curves. A half-cell configuration was used, and CE performances were compared to CEs made of carbon black (CB) and Pt. A CE assembled with HS-SWNH and mixed with 10wt.% of hydroxyethyl cellulose (HEC) - HS-SWNH/HEC was found to have the highest electrocatalytic activity (lowest R-ct) among all the carbon-based CEs tested when annealed at 180 degrees C (R-ct=141 cm(2)); however, a very thick film (several tens of mu m) would be required in order to perform comparably to a Pt CE. The annealing of such CE at higher temperatures (above 400 degrees C) did not improve its catalytic activity, contrary to the other studied carbonaceous CEs. The redox catalytic activity of SWNH and HS-SWNH decorated with Pt was also studied on a half-cell configuration and compared to that of Pt/CB and pristine Pt. The Pt/SWNH/HEC CE showed the highest electrocatalytic activity per mass of Pt, needing just 50% of Pt load to yield the same electrocatalytic activity of a DSC equipped with a Pt CE, but having half of its transparency. Additionally, applications in temperature-sensitive substrates are envisioned for the Pt/SWNH/HEC CE due to the use of lower annealing temperatures. Copyright (c) 2012 John Wiley & Sons, Ltd.