The electronic absorption spectrum of single-walled carbon nanohorn (SWCNH) shows a relatively broad absorption peak at 256-259 nm when suspended in tetradecane and in decalin. The electronic absorption spectrum is also characterized by a series of small bands recalling the spectra of C-60 and C-70 fullerenes. Surprisingly, SWCNH shows a minimal solubility in 1,2,4-trichlorobenzene. SWCNH is also dissolved and oxidized in oleum (fuming sulphuric acid with 20% free SO3), displaying a radical cation band in the near infrared at 1045 nm. The addition of ozone to a suspension of SWCNH in oleum enhances its dissolution and the relative radical cation absorption band.