A purification process combining both a gas-phase purification using H@sub 2@S and O@sub 2@ mixture to remove impurity carbon particles and an acid treatment to remove metal particles has been developed for single-wall carbon nanotubes (SWNTs), synthesized by an arc-discharge method. Acid solution of 3 M HCl was used to remove catalyst metal particles out of the as-grown SNWTs. Thermal oxidation using the H@sub 2@S-O@sub 2@ gas mixture was followed to preferentially remove unwanted carbon particles. Hydrogen sulfide played a role of enhancing the removal of carbon particles as well as controlling the oxidation rate of oxygen with carbon. The optimum gas ratio was 10 ml H@sub 2@S/min and 20 ml O@sub 2@/min. After the acid treatment and thermal oxidation at 500 °C for 1 h, we achieved carbon nanotubes purity of > 90 % with yield of 20 - 50 %, depending on the purity of starting material.