Closed end single walled carbon nanotubes (c-SWNT) purified by acidic oxidation have been studied in order to determine the fraction of oxidized carbon sites present. This has been done by measuring the evolution of CO2(g) and CO(g) on heating to 1273 K. Following a 1273 K treatment, the defect sites were titrated repeatedly with O3. The results indicate that ~5% of the carbon atoms in the c-SWNT samples pretreated to 1273 K are located at defective sites, capable of facile oxidation by O3. This high defect site density indicates that a significant number of these sites exist on the graphene walls of the nanotubes.