Reductive alkylation of SWNTs using lithium and alkyl halides in liquid ammonia was found to yield sidewall functionalized nanotubes that are soluble in common organic solvents. Atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) of dodecylated SWNTs prepared from HiPco nanotubes show that extensive debundling has occurred. SWNTs functionalized by dodecyl groups can be used for length measurements by atomic force microscopy. Different thermal behavior of the dodecylated SWNTs was observed when the reductive alkylation was carried using either sodium or potassium. Solid state ¹³C NMR spectroscopy indicates that the product derived using sodium may have a higher proportion of 1,4- than 1,2-addition of the alkyl groups. 1,4-Addition products are expected to exhibit greater thermal stability. SWNTs functionalized by phenyl groups can be sulfonated in oleum (H₂SO₄, 20% free SO₃) to give material that exhibits high solubility in water. Cytotoxicity studies show that SWNTs with a high degree of sidewall functionalization are not toxic to skin cells.