We investigated noncovalent molecular adsorptions onto carbon nanotubes (CNTs), using density-functional theory methods including recently developed hybrid functionals. Planar aromatic molecules have greater binding strength than fully saturated nonaromatic molecules, and adsorption geometries are consistent with the features of intermolecular π -π stacking. We show that modifications with alcohol or a thiol terminal group increase the adsorption strength consistently throughout the considered molecules. This result may be used in experiments involving noncovalent sidewall functionalizations of CNTs.