Multiple-walled carbon nanotubes (MWNTs) produced using the chemical vapor deposition method were functionalized via attaching aminopolymer poly(propionylethylenimine-co-ethylenimine) to the nanotubes. Two different reaction conditions based on acylating the nanotube-bound carboxylic acids and on directly heating nanotubes in the polymer melt were used and compared. Both methods were effective in the nanotube functionalization, and the polymer-attached MWNTs were found to be soluble in many common organic solvents and in water. Results from the characterization of the functionalized nanotube samples using electron microscopy, optical spectroscopy, NMR, and thermal analysis techniques are presented and discussed.