Single-walled carbon nanotubes (SWNTs) were dissolved in a basic aqueous (NaOH) solution through the formation of a complex with folic acid. Water-solubilisation of SWNTs by folic acid is superior to that by guanosine 5′-monophosphate (GMP). The addition of diluted hydrochloric acid to the folic acid–SWNT mixture resulted in the precipitation of SWNTs from the basic solution. The precipitate was then redissolved by the addition of the basic solution without the need for sonication. Although the solubilisation of SWNTs was also controllable by changing temperature, sonication for 2 h was required to redissolve the SWNTs.