Single-walled carbon nanotubes (SWCNTs) were dispersed in an aqueous solution of a long-chain amidoamine surfactant (C18AA) at a high concentration (>0.5 wt%). The SWCNT dispersion was very stable in the pH range from 7.0 to 11.5. Addition of chloroform into the aqueous dispersion at pH 11.5 brought about the precipitation of SWCNTs in water, although there was no effect on the dispersibility of SWCNTs at pH 7. We demonstrated that reversible dispersion–precipitation regulation of SWCNTs in water can be achieved by changing pH and adding the C18AA surfactant in a biphasic system of C18AA–water–chloroform.