Control of the conductivity of single wall carbon nanotubes (SWNTs) is crucial for the use of carbon nanotubes in molecular electronics. We report a new fundamental characteristic of semiconducting SWNTs: the persistent photoconductivity of chemically modified carbon nanotube films. Illumination of carboxylated semiconducting SWNTs with ultraviolet or visible light causes a persistent decrease in the conductivity of semiconducting films. The photoinduced conductivity persists in the dark with a characteristic half-life of 35 s to 1.2 x 10(3) s at room temperature and an activation energy of 0.35 eV. Infrared illumination restores the conductivity of SWNT films. Ultraviolet and visible light illumination partially refills empty valence band states of chemically modified SWNTs by electron injection from the dopant sites. Photoinduced injection of electrons is accompanied by a decrease of the conductivity of the p-doped SWNT film, because of neutralization of holes by injected electrons. Covalent attachment of ruthenium(II)-tris(2,2'-bipyridine) (Ru(bpy)(3)(2+)) to SWNTs makes carbon nanotubes sensitive to light that has been absorbed by the ruthenium complex and makes the carbon nanotubes persistently photoconductive. The photoconductivity of Ru(bpY)(3)(2+)-SWNT films is presumably due to the injection of holes from *Ru(bpy)(3)(2+) to SWNT with a quantum yield of 0.55. Persistently photoconductive SWNTs have potential uses as nanosized optical switches, photodetectors, electrooptical information storage devices, and chemical sensors.