A versatile and highly efficient zwitterion method to functionalize single-walled carbon nanotubes (SWCNTs) as well as fullerenes has been developed. This approach employs a combination of moderately weak electrophiles and nucleophiles to create reactive zwitterions, which are the key intermediates toward the functionalization. Such novel protocol provides a powerful platform for functionalization of SWCNTs and fullerenes in a modular fashion and will be of great use to both polymer chemistry and materials science.