Single-walled and multiple-walled carbon nanotubes were functionalized with a polystyrene copolymer, poly(styrene-co-p-(4-(4‘-vinylphenyl)-3-oxabutanol)). The functionalization reaction conditions were designed for the esterification of the nanotube-bound carboxylic acids. The polymer-attached carbon nanotubes are soluble in common organic solvents, making it possible to characterize the samples using not only solid-state but also solution-based techniques. The solubility has also allowed an intimate mixing of the functionalized carbon nanotubes with polystyrene. Results from the characterization of the functionalized carbon nanotubes, including the chemical and thermal defunctionalizations of the soluble samples, and the fabrication of polystyrene−carbon nanotube composite thin films using a wet-casting method are presented and discussed.