The opening of bud-type single-wall carbon nanohorns (SWNHs) by heat treatment in CO2 was studied by TEM and nitrogen adsorption (77 K). The adsorption isotherms indicate that oxidation by CO2 at 1273 K provides a sufficient nanohorn opening. The pore structure parameters of the open nanohorns can be controlled by varying the treatment conditions; the size of the generated nanopores increases with the temperature and time of treatment. In addition, TPD experiments indicate a significant decrease of the oxygen content in SWNHs opened by heat treatment in CO2. This is in contrast to the procedures reported in the literature for opening of carbon nanotubular structures, typically introducing oxygen functional groups.