Single wall carbon nanotubes (SWCNTs) and vapor growth carbon fibers (VGCFs) were chemically modified by radical reactions with 2,2'-azobis(isobutyronitrile) (AIBN) and 4,4-azobis(4-cyanovaleric acid) (ACVA) in o-dichlorobenzene. The modification was monitored by infrared and Raman spectra and differential thermal gravimetry. These spectral and analytical data supported the chemical modification of SWCNTs and VGCFs by a simple and clean procedure. AIBN-modified SWCNTs and VGCFs were well dispersed in organic solvents such as methanol, tetrahydrofuran, and chloroform. In contrast, ACVA-modified SWCNTs and VGCFs were dispersed in water.