A series of highly efficient, modular zwitterion-mediated transformations have been developed which enable diverse functionalization of carbon nanotubes (CNTs, both single-walled and multi-walled) and fullerenes. Three functionalization strategies are demonstrated. (1) Trapping the charged zwitterion intermediate with added nucleophiles allows a variety of functional groups to be installed on the fullerenes and carbon nanotubes in a one-pot reaction. (2) Varying the electrophile from dimethyl acetylenedicarboxylate to other disubstituted esters provides CNTs functionalized with chloroethyl, allyl, and propargyl groups, which can further undergo S(N)2 substitution, thiol addition, or 1,3-dipolar cycloaddition reactions. (3) Postfunctionalization transformations on the cyclopentenones (e.g., demethylation and saponification) of the CNTs lead to demethylated or hydrolyzed products, with high solubility in water (1.2 mg/mL for MWCNTs). CNT aqueous dispersions of the latter derivatives are stable for months and have been successfully utilized in preparation of CNT-poly(ethylene oxide) nanocomposite via electrospinning. Large-scale MWCNT (10 g) functionalization has also been demonstrated to show the scalability of the zwitterion reaction. In total we present a detailed account of diverse CNT functionalization under mild conditions (60 degrees C, no strong acids/bases, or high pressure) and with high efficiency (1 functional group per 10 carbon atoms for SWCNTs), which expand the utility of these materials.