Two common methods of oxidation, gas-phase oxidation by heat treatment in oxygen gas and liquid-phase oxidation using nitric acid, were applied to single-wall carbon nanotubes (SWNT). The heat treatment in oxygen showed that thinner SWNTs burn more quickly. The nitric acid treatment showed that SWNTs are relatively inert to oxidation using acids. When the nitric acid treated samples were further oxidized by heat treatment in oxygen, selective oxidation of thinner SWNTs occurred.