A purified single-walled carbon nanotube (SWNT) sample was fractionated in sequential functionalization reactions with a diamine-terminated oligomeric poly(ethylene glycol) to produce five soluble fractions and a final solid residue. These fractions and the residue were characterized by using optical spectroscopy, electron microscopy, thermal gravimetric analysis, and other techniques. The results show that the soluble fractions all contain primarily functionalized SWNTs and that there are no obviously systematic changes from one fraction to the next. However, one exception made evident by the Raman spectroscopy results is that the functionalization reactions preferentially solubilize the SWNTs of smaller diameters, resulting in an enrichment of larger diameter nanotubes in the final solid residue. In addition, the nanotube contents in different soluble fractions obtained from the sequential functionalization reactions are apparently different, with higher nanotube contents in the later fractions. The difference between the fractionation of SWNTs and the fractionation of multiple-walled carbon nanotubes (MWNTs) reported previously is discussed.