In this work, seven types of surface-modified single-walled carbon nanotubes (SWNTs) were studied by X-ray photoelectron spectroscopy (U'S) and Raman spectroscopy to investigate the functional groups and extent of functionalization. Hansen solubility parameters were determined based on observations of the sedimentation and swollen states of the SWNTs in solvents after ultrasonication, and the results were compared with the hydrodynamic sizes of the SWNTs evaluated by the dynamic light scattering method. We found that the solubility of SWNTs is related to their functional groups and degree of functionalization, as well as their size.