X-ray diffraction (XRD) and electrical resistance measurement on single-walled carbon nanotube (SWNT) samples prepared by the are-discharge method are reported. The XRD profile of heat-treated sample indicated that air (oxygen, and/or nitrogen and/or water) can be condensed inside the SWNTs. We also found that the electrical resistance of SWNT soot is significantly affected by exposing to the oxygen gas and humid air.