With the goal of functionalizing carbon nanotubes for possible applications in biological and biomedical environments, we investigated the interactions between perfect and defective single walled carbon nanotubes (SWCNTs) and phosphate groups using density functional theory (DFT). The phosphate groups included phosphoric acid (H 3 PO 4 ) and dihydrogen phosphate (H 2 PO 4 ). Defects included monovacancy, divacancy, and Stone-Wales (SW) defects. Our results revealed that with H 3 PO 4 , functionalization occurs by an electrostatic interaction, and no significant changes were observed in the band structures or in the band gap. With H 2 PO 4 , functionalization occurs through C–O interaction, and is mainly favored in the presence of a monovacancy or SW defect. With a monovancy defect, the nanotube preserves its sp 2 hybridization, whereas with divacancy and SW defects, local sp 3 hybridization occurs. Functionalization with H 2 PO 4 resulted in changes in the band structures and the band gap of perfect and defective (10, 0) SWCNTs. These changes are caused by electronic localization states on the Fermi level, but the semiconductor behavior of nanotubes is preserved.