Communication: The solubility properties of single-walled carbon nanotubes (SWNTs) and the factors that control the dissolution of these materials have been investigated in detail. The authors report a number of strategies that lead to soluble SWNTs, in particular the covalent attachment of long alkyl chains to the carboxylic acid-terminated SWNTs shown in the Figure (produced by the purification process) via formation of amides and by formation of zwitterions by an acid-base reaction.