Polyethylene glycol (PEG) functionalized double-walled carbon nanotubes (DWNTs) have been synthesized by a [2 + 1] cycloaddition reaction and characterized by transmission electron microscopy, atomic force microscopy, Raman spectroscopy, thermal gravimetric analysis, and UV–visible spectroscopy. Functionalization affords a large increase in the aqueous solubility of DWNTs. The saturated concentrations of DWNTs functionalized with diazido-terminated PEG₈₀₀ (with a molecular weight of 800) and azido-terminated PEG₇₅₀ monomethylether (with a molecular weight of 750) are very similar—0.36 and 0.37 mg/mL (DWNTs equivalent concentration), respectively.