The pore size distributions of bundled single-walled carbon nanotubes were investigated by N-2 adsorption and desorption. It was found that the pore sizes are in two main ranges: <5 nm and 10-100 nm. Furthermore, their heating in an inert gas, CO2, or H-2 at 800 degreesC did not change the pore size distribution, even though significant amounts of nanotubes were consumed during the latter two treatments. In contrast, the nitric acid treatment at 70 degreesC completely changed the pore size distribution, generating an active-carbon-like pore size distribution and a large increase of the total surface area.