The oxidative etching of single walled carbon nanotubes (SWNTs) is useful for opening the end caps and the sidewalls of the nanotubes to provide access to the interior. We have studied the effect of successive treatment of SWNTs by 95% pure O-3(g), which is a powerful and convenient oxidizing agent. The surface area of the SWNTs was measured following exposure to O-3(g) at 300 K and also following heating to 1073 K in a vacuum to decompose the oxidized groups on the nanotubes, a procedure called etching. This O-3-induced etching process was observed by scanning electron microscopy and by transmission electron microscopy, and the kinetics of the process was studied gravimetrically. It was found that O-3 attack occurs preferentially on the outermost geometric surface of the conglomerate sample of the nanotubes as a result of the high efficiency of O-3 to react in a few collisions with the nanotube surface. Ozone-induced etching causes the loss of pores in the 20 angstrom diameter range as observed by nitrogen adsorption at equilibrium by density functional theory analysis.