Electron transport properties of single-walled carbon nanotubes are investigated with TTF and TCNQ molecules, i.e., tetrathiafulvalene and 7,7,8,8-tetracyano-p-quinodimethane. TTF and TCNQ are well known strong charge transfer molecules. From the current-voltage measurement in a vacuum, current decreases and increases distinctly upon the adsorption of TTF and TCNQ, respectively. The same results are observed in the case of MAT-single-walled carbon nanotube (SWNT) device, representing a good prospect for future nanostructured devices using carbon nanotubes, especially as chemical sensors. According to the spectroscopic data from x-ray spectroscopy and near infrared spectroscopy, the adsorbed TTF and TCNQ molecules act as electron-donating and withdrawing species, respectively, on the surface of SWNTs. (C) 2004 American Vacuum Society.